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    ETHYLENETHIOUREA (ETU)

    First draft prepared by
    A. Kocialski, Office of Pesticide Programs,
    US Environmental Protection Agency, Washington, DC, USA

    EXPLANATION

         ETU was reviewed in conjunction with the ethylene bis
    dithiocarbamates (EBDCs) by the Joint Meeting in 1963, 1965, 1967,
    1970, 1974, 1977, 1980, 1986, and 1988 (Annex I, references 2, 4, 8,
    14, 22, 28, 34, 47 and 53).  In 1988, the Joint Meeting extended the
    temporary ADI of 0-0.002 mg/kg bw pending the submission of
    additional data.  ETU is also of interest because it forms part of
    the terminal residue to which consumers of produce treated with the
    EBDCs are exposed and because the levels of ETU in treated produce
    generally increase during food processing as the levels of the EBDC
    parent compounds decrease.

         This monograph summarizes new or not previously-reviewed data
    on ETU as well as relevant data on this substance from previous
    monographs and monograph addenda on the EBDCs.

    BIOLOGICAL DATA

    Biochemical aspects

    Absorption, distribution, excretion, and biotransformation

    Mice

         Twenty-one adult male ND/4(S)BR mice were divided into groups
    that received oral doses of 0.05 or 0.25 mmol/kg bw of either 14C-
    ETU (99% pure), ethylenebis(isothiocyanate) sulfide (14C-EBIS, 99%
    pure), or 14C-labelled maneb or zineb (purity not stated).  EBIS,
    ETU, EU (ethyleneurea) and other products were absent from maneb and
    zineb.  Pooled 0-24 and 24-48 hour urine samples were analyzed for
    radioactive products.  None of the administered compounds was
    excreted as radiolabelled CO2.  Essentially all of the ETU was
    recovered in excreta within 48 hours. Approximately 10% of the
    radioactivity from maneb and zineb was excreted in the urine whereas
    between 40 and 70% of the EBIS radioactivity and about 50% of the
    ETU radioactivity was excreted in the urine, approximately half of
    which was unchanged ETU.  Approximately 12% of the radioactivity
    excreted in the urine following ETU administration was EU, with the
    remainder being unidentified polar products.  The administration of
    EBIS at the lower dose produced 99.7% unidentified polar products
    while at the higher dose, ETU and EU were each present at 10%, and,
    polar products were reduced by 25%-76%.  ETU in urine amounted to
    0.5% and 1.3% of the administered high- and low-dose of maneb,
    respectively.  Following the 0.25 mmol/kg bw dose of zineb, 1% of
    the radioactivity was present in the urine as ETU.  The majority of
    the radioactivity in the urine of mice given maneb or zineb was
    present as unidentified polar products.  No EBIS was detected in the
    urine of mice given maneb or zineb (Jordan & Neal, 1979).

         Two weeks prior to breeding, four female C57BL/6N mice were
    administered ETU (96.5% purity) in the diet at dose levels of 0, 33,
    100, 333, or 1000 ppm.  During the gestation period the level of ETU
    equivalents in amniotic fluid, placenta and fetal carcass correlated
    with maternal blood levels; however, levels were increased in
    maternal livers (3 times).  No differences between dosed dam and
    fetuses were observed.  In the post-partum period, accumulation of
    ETU equivalents was much more apparent, with ETU equivalents in
    maternal liver approximately 10 times greater than maternal blood. 
    Levels of ETU equivalents were also increased 2 times in maternal
    milk compared to maternal blood.  Levels in maternal milk were 13
    times neonatal blood levels.  Neonatal liver and blood significantly
    correlated with regard to ETU equivalents.  Pretreatment did not
    alter the pharmacokinetics of ETU in post-partum dams or their
    neonates (Peters  et al., 1982).

    Mice/rats

         A dose of 240 mg/kg bw of ETU (> 98% purity) was administered
    via stomach intubation to pregnant mice and rats on day 15 of
    gestation.  Radiolabel concentrations peaked in mice and rats at
    approximately the same time, 1.3 and 1.4 hours after dosing,
    respectively, and maternal and fetal tissue levels were similar 3
    hours after treatment.  Thereafter levels in mouse tissues (maternal
    and fetal) declined more rapidly.  The half-lives for ETU
    elimination from maternal blood were 5.5 and 9.4 hours in mice and
    rats, respectively.  The main route of excretion was via the urine
    with 74% and 70% of the applied dose excreted by the mouse and rat,
    respectively, in 48 hours (Ruddick  et al., 1977).  In mice, ETU
    comprised 40% of labelled metabolites in urine versus 95% in rat. 
    This suggested more rapid metabolism in the mouse than in the rat. 
    The major urinary metabolite identified in the mouse was 2-
    imidazolin-2-yl sulfemic acid from the oxidation of ETU (Savolainen
    & Pyysalo, 1979).

    Rats

         Male Sprague-Dawley rats (4/group) received single dermal
    applications of 2.6, 26, or 260 g 14C-ETU/rat.  Ten hours after
    application the treatment area was wiped, excreta (urine/faeces)
    were collected and animals were sacrificed.  Application sites
    (skin) were removed and analyzed for 14C-label.  Whole blood,
    plasma, thyroid, liver along with additional organs/tissues and the
    remaining carcass were collected and analyzed.  Another set of male
    Sprague-Dawley rats (8/group) received single applications of 0 or
    2.6 g 14C-ETU/rat by either the oral, dermal, or intravenous
    route.  Initial blood samples were drawn at 5 and 30 minutes in
    animals administered by the i.v. route and oral/dermal routes,
    respectively.  Initial collection of excreta began at 10 hours after
    application.  All samples were collected on a predetermined schedule
    through termination at 7 days.  The skin and carcass were examined
    for 14C content.  One group receiving dermal exposure had the
    treatment area wiped at 10 hours, the second groups at 7 days post-
    administration.

         Animals receiving i.v. and oral administration showed 100 and
    91% total absorption, respectively, with at least 85% (oral)
    appearing in the excreta.  ETU was primarily excreted in the urine
    within 24 hours.  The percent absorption of ETU for animals
    receiving 2.6 g/rat, swabbed (wiped) at 10 hours and terminated at
    10 hours or 7 days was 17% and 26%, respectively.  Animals receiving
    2.6 g/rat and left unwiped for 7 days showed 53% absorption. 
    Animals receiving 26 and 260 g/rat, wiped at 10 hours and
    immediately sacrificed recorded absorption values of only 5 to 6%. 
    Total recovery of 14C-label ranged from 80% to 100% for all routes. 

    The amount of applied material remaining at the skin site after 10
    hours of exposure was about 40% for animals receiving 2.6 and 26 g
    and about 13% at 7 days after wiping at 10 hours.

         At a single dose of 2.6 g-ETU, 70%-90% was excreted within 24
    hours, mostly in the urine (40-65%).  Cumulative total 14C-
    excretion in male rats following 10 hour dermal exposure with 2.6 g
    ranged between 6 and 13% at 24 hours and between 13 and 22% at 24
    hours.  Cumulative excretion after wiping at 10 hours indicated that
    test material bound to the skin continued to be absorbed to some
    degree.  At 7 days, total cumulative excretion was 20-28%.  The
    majority of the excretion occurred in the urine.  Tissue
    concentration 10 hours after administration for the 12 tissues
    examined ranged from 0 ppm to 0.457 ppm with the greatest amount
    concentrating in the thyroid (0.457 ppm), which represented 0.01% of
    the 260 g administered.  Animals receiving the lower doses of 2.6
    and 26 g/rat of ETU showed no thyroid accumulation of ETU.  The
    limit of detection for ETU in the thyroid was approximately 0.020
    ppm. The lack of 14C accumulation in the thyroid at 26 g/rat and
    lower doses could be explained in part by the fact that the data
    appeared to indicate that a finite amount (approximately 0.09 g) of
    ETU or its metabolites was preferentially bound to red blood cells
    (DiDonato & Longacre, 1987).

         Either 2-14C-ETU or 4,5-14C-ETU (> 98% purity) was
    administered to four pregnant Wistar Imamichi rats at 100 mg/kg bw
    via intra-gastric intubation on the 12th day of gestation.  Whole
    body radiography, TLC and GC were used to analyze the uptake of
    radioactivity in tissues of both the fetus and the dam.
    Radioactivity in the fetus reached maximum activity within 2 hours
    and declined thereafter.  Differences were observed between 2-ETU
    and 4,5-ETU with respect to protein fraction incorporation. 
    Radioactivity was distributed homogeneously throughout all tissues
    except for the thyroid, where there was an increase in activity
    during the first 24 hours.  Thyroid hormones are reported to play
    important roles in the development of the CNS and thyroidectomy
    induces malformations in the rat.  There was no significant
    difference in the T4 levels between treated and control maternal
    serum, whereas the appearance of malformed fetuses was significant
    at 100 mg/kg bw (malformations were observed in 100% of the fetuses
    from treated dams) (Kato  et al., 1976).

         Wistar female rats were treated with single oral doses of 240
    mg/kg bw ETU (purity not specified) and 25 or 50 Ci/kg bw of 14C-
    ETU (99% pure) on days 11 or 12 of gestation and sacrificed at 6, 12
    or 24 hours post-treatment.  Radioactivity in maternal kidney,
    liver, blood and urine as well as pooled embryos was determined. 
    Additional animals dosed on day 15 were sacrificed at 3 hours post-
    dosing and in addition to the above tissues, the muscle and placenta
    were analyzed for radioactivity.  Blood levels were determined 

    at 0.5, 1, 2, 4, 6, 12, 24 and 48 hours.  Urine analysis was
    conducted at 12, 24, 32 and 48 hours.  The binding of ETU to
    maternal RBCs was also studied, as well as the binding to embryonic
    tissues (DNA, RNA) in pooled day-12 embryos at 6 and 12 hours post-
    treatment.  Metabolites in urine were also investigated.

         The distribution of radioactivity in maternal tissues (kidney,
    liver, blood) was essentially the same at 6 and 12 hours post-dosing
    on days 11 or 12, but the level decreased 80-90% at 24 hours.  The
    distribution of radioactivity in fetal tissue on days 11 or 12 at 6
    and 24 hours was generally comparable.  However, at 12 hours day 12
    values were decreased approximately 50%, whereas the day 11 value at
    12 hours was unchanged from the 6-hour reading.  The distribution of
    radiolabel in maternal tissues was 1.2-2.5 times greater at all time
    periods at days 11 and 12.  Radioactivity in urine was similar at
    all times examined on days 11 and 12 of gestation.  Radioactivity
    levels in maternal liver, kidney, muscle and placenta and the fetus
    at 3 hours post-administration on day 15 of gestation were similar. 
    The maternal blood half-life was calculated to be about 10 hours.

         The radioactive label, which was weakly bound to metabolites,
    was distributed uniformly between RBCs and plasma of maternal blood. 
    No radioactive label was detected in DNA, RNA or the protein
    fractions of embryonic tissues.  Metabolites in maternal urine
    generally indicated the same pattern at all treatment times -
    primarily ETU with traces of ethyleneurea and 2 unidentified
    metabolites (Ruddick  et al., 1976).

         Two weeks prior to breeding, four female Fischer 344 rats were
    administered ETU (96.7% purity) in the diet at dose levels of 0, 8,
    25, 83 or 250 ppm.  During the gestation period the amount of ETU
    equivalents measured in maternal liver, amniotic fluid and fetal
    carcass correlated with the maternal blood level, but the placental
    levels did not.  Transplacental transport was demonstrated.  Post-
    partum, there was an apparent transfer of ETU to the nursing pups
    via the milk.  Levels of ETU equivalents in maternal liver, maternal
    milk, neonatal blood and neonatal liver were increased compared to
    maternal blood levels.  There were no significant differences,
    however, between ETU equivalents in maternal milk and levels in
    neonatal blood. No accumulation of ETU in neonatal liver or maternal
    liver was observed.  The level of ETU in neonatal liver correlated
    with the levels in neonatal blood.  Prior exposure of maternal
    animals to ETU did not affect the pharmacokinetic behaviour of ETU
    in post-partum animals (dam and neonate) (Peters  et al., 1982).

         Approximately 80-82% of a single oral 4 mg/kg bw dose of 14C-
    ETU (> 99% purity) was eliminated via the urine within 24 hours by
    three male Sprague-Dawley rats.  A half-life of 5.6 hours in rat

    blood was demonstrated.  Unchanged ETU represented 62.6% of the
    radioactivity in rat urine.  Metabolites included EU (18.3%),
    imidazolone (4.9%) and imidazoline (1.9%) (Iverson  et al., 1980).

    Rats/guinea-pigs

         Six male adult Wistar rats and six male Hartley guinea-pigs
    were fasted for 24 hours prior to the administration of 20 mg/kg bw
    ETU (purity not given) by oral intubation as a single dose.  Food
    and water were withheld for 5 hours post-dosing.  Urine and faecal
    samples were collected and animals sacrificed at 96 hours post-
    dosing.  Liver, kidney, heart, thyroid and muscle were excised and
    frozen.  ETU analysis was carried out by gas-liquid chromatography. 
    Recovery was 90% or greater.  The limit of detection was 0.005 ppm
    of ETU.  At 24 hours post-dosing 60% of the administered dose was
    excreted unchanged in the urine of rats and 44% in the urine of
    guinea-pigs.  Urinary excretion of ETU was complete at 72 hours
    (64%) in rats and at 48 hours (46%) in guinea-pigs.  Rats eliminated
    1.1% of the administered dose in faeces while guinea-pigs eliminated
    0.8% in faeces within a 48-hour period.  Mean residue levels in
    liver, kidney, heart and muscle ranged from 0.01 ppm to 0.086 ppm. 
    Thyroid concentrations of ETU in rats and guinea-pigs were 0.82 and
    0.75 ppm, respectively (Newsome, 1974).

    Guinea-pigs

         The backs of 12 male Hartley guinea-pigs were shaved and the 12
    animals divided into 2 groups of 6 each.  The epidermis of the backs
    of one group was abraded.  ETU (99% pure; 15 mg/ml) was applied to
    both groups over an area of 40 X 40 mm, which was then covered with
    non-woven fabric.  After 24 hours three animals of each group were
    sacrificed and the distribution of radioactivity as a percent of the
    applied dose was determined for blood, certain internal organs,
    faeces, urine, skin, the fabric covering and the rinse wash from the
    application site.  The remaining animals were sacrificed at 24 hours
    and radioactivity was determined in a number of tissues.  Another
    group of 25 Hartley male guinea-pigs was dosed orally with a single
    dose of 5 mg/kg bw ETU.  Five animals were sacrificed 1, 3, 6, 24,
    or 48 hours after dosing, and the concentration of radioactivity was
    determined in a number of tissues.  Additionally, urine was
    withdrawn directly from the urinary bladder 2 hours after oral
    administration, after which urinary metabolite determinations were
    made.

         Absorption of ETU from intact and abraded skin was 14 and 42%,
    respectively, at 24 hours.  The highest concentration of
    radioactivity was found in the thyroid, which was at least 10 times
    greater than in any other tissue. One hour after oral
    administration, the radioactivity was distributed evenly among all
    organs and tissues except adipose tissue.  At 48 hours, most of the

    radioactivity had cleared from all the organs and tissues except the
    thyroid.  The radioactive half-life was 13 hours in the liver, about
    7.5 hours in the kidney and blood, and about 42 hours in the thyroid
    gland.  Nearly 80% of the administered radioactivity was excreted in
    the urine within 48 hours and about 10% was recovered in the faeces. 
    Thin-layer chromatograms of urine collected from the urinary bladder
    indicated that ETU was the primary metabolite (93%), with about 7%
    of the parent compound converted to unidentified but strongly polar
    metabolites (Teshima  et al., 1982).

    Cats

         Approximately 80-82% of a single oral 4 mg/kg bw dose of 14C-
    ETU (> 99% purity) was eliminated via the urine within 24 hours by
    3 female cats.  The half-life was 3.5 hours in blood.  Unchanged ETU
    represented 28% of the radioactivity in urine, while S-methyl ETU
    comprised 64% of the radioactivity in urine (Iverson  et al.,
    1980). 

    Rats/monkeys

         Four female Sprague-Dawley rats and two adult female rhesus
    monkeys were given 14C-ETU (99% pure) by stomach tube at a dose of
    40 mg/kg bw in a water vehicle as a single dose.  Animals were
    housed singly in metabolism cages and the excreta were collected
    over a 48-hour period, after which time the animals were sacrificed
    and all tissues, including skin, muscle and bone were weighed. 
    Representative samples from each tissue were oxidized and counted on
    a scintillation spectrometer.  Additional samples of tissue were
    embedded in paraffin and sectioned followed by staining with
    haematoxylin and eosin.

         No gross or microscopic changes were observed in either
    species.  Urinary excretion at 48 hours in monkeys ranged between 47
    and 64% of the total administered radioactivity, while it averaged
    82% in rats.  Less than 1.5% was found in the faeces of both
    species. Total tissue distribution (i.e. total body burden at 48
    hours) ranged between 21 and 28% of the administered dose in monkeys
    and less than 1% in rats.  Muscle, blood, skin and liver contained
    12, 2.8, 2.4 and 1.0%, respectively, of the initial dose in monkeys
    and less than 0.3% in rats.  One female monkey had a slightly higher
    concentration of 14C in the thyroid compared to other tissues
    within the same animal.  Two rats had higher 14C activity in the
    thyroid gland than in other tissues (Allen  et al., 1978).

    Monkeys

         Male  Macaca mulatta (rhesus) monkeys were given 2-3 mg/kg bw
    14C-ETU by oral gavage. Whole blood and excreta (urine and faeces)
    were collected and examined.  Radioactivity peaked in blood at 8
    hours and declined rapidly at 24-48 hours.  Approximately 50% of the
    dose was excreted in urine within 24 hours.  Less than 1% of the
    dose was recovered in faeces during the first 24 hours, and none
    thereafter (Emmerling, 1978a).

         The pathways of ETU metabolism in mice, rats and cats are given
    in Figure 1.

    Effects on enzymes and other biochemical parameters

    Mice

         Liver microsomes were prepared from 3- and 30-week old male and
    female Swiss-Webster mice to determine relative flavin monooxygenase
    (FMO) activity and cytochrome P-450 activity via oxidation of N,N-
    dimethylaniline and N-demethylation of dimethylaniline,
    respectively.  Enzyme activity related to ETU metabolism and binding
    was also evaluated.  FMO activity was significantly lower in older
    males than in young males.  No such differences were observed in
    comparisons between females.  N-Demethylase activity was not
    affected by age, sex, or sensitivity to the heat denaturation
    effects to FMO.  ETU metabolism was similar in young and older
    females, but it was significantly lower in older males than in young
    males.  FMO-dependent activity accounted for 75% of the total
    binding in all animals, but microsomes from older males bound
    significantly less radioactivity (30%) than those from young males
    (Hui  et al., 1988).

    Mice/rats

         Possible qualitative differences in the metabolism of ETU
    between mice and rats have been noted on the basis of urinary
    metabolites and measurement of microsomal enzymes.  Microsomal
    enzymes (aminopyrine N-demethylase, aniline hydroxylase, and
    cytochrome P-450) were inhibited in rats, whereas in mice they were
    stimulated. This suggests that ETU is metabolized by different
    enzymatic pathways in the two species (Lewerenz & Plass, 1984).

    FIGURE 01

    Rats

         Male Sprague-Dawley rats were pre-treated with phenobarbital,
    dexamethasone, beta-napthoflavone or left untreated.  The  in vitro
    effect of ETU or EU in the presence and absence of glutathione,
    NADPH and heat inactivated microsomes on P450 enzymatic activity and
    on covalent binding of ETU to microsomal proteins was studied.  ETU
    inhibited P450 activity in pretreated and non-pretreated rats. 
    Inhibition was NADPH-dependent and was abolished by glutathione
    (GSH).  Covalent binding of 14C-ETU to microsomal protein was also
    NADPH-dependent.  Binding was inhibited by co-incubation with GSH. 
    Heat treatment of microsomes and P450 inactivation studies indicated
    a prominent role of FMO in covalent binding.  Addition of GSH or
    dithiothreitol after incubation of microsomes resulted in release of
    bound ETU.  Metabolism of ETU in the presence of GSH resulted in the
    formation of GSH-ETU adducts and subsequent disulfide exchange.  The
    results suggest that reactive metabolites from ETU generated by
    either FMO or P450 are trapped by GSH.  Initial oxidation of ETU to
    imidazoline-2-sulfenic acid, primarily by FMO, followed by reaction
    with GSH or protein sulfhydroyls under conditions of GSH depletion,
    has been proposed as the route of monooxygenase-mediated metabolism
    of ETU (Decker & Doerge, 1991).

         Male Sprague-Dawley rats were divided into 3 groups in a study
    designed to study the effect of ETU on RNA synthesis.  One group
    that had been fasted for 16 hours received single i.p. injections of
    2.5 or 250 mg/kg bw ETU or 5.0 mg/kg bw thioacetamide, followed by
    3H-orotic acid 60 minutes later.  Control animals received dimethyl
    sulfoxide alone.  A second group received 5.0 or 250 mg/kg bw/day
    ETU by gavage on 3 successive days followed by administration with
    3H-orotic acid; the animals were killed on the fourth day.  The
    third group was administered 5.0 or 250 ppm ETU in the diet for 3
    weeks.  At the end of 3 weeks, a 1-hour pulse dose of 3H-orotic
    acid was administered and the animals killed.  Serum T4 levels in
    animals given ETU were then determined by radioimmunoassay.  Rats
    receiving 400 ppm of acetylaminofluorene in the diet served as
    positive controls.

         The livers of all animals given either ETU or thioacetamide
    were histologically normal at the time of sacrifice.  The livers of
    rats fed acetylaminofluorene showed mild hydropic change of
    hepatocytes and minimal bile duct proliferation.  ETU failed to
    inhibit nuclear or cytoplasmic RNA synthesis under the test
    conditions.  However, thioacetamide and acetylaminofluorene both
    reduced the incorporation of 3H-orotic acid into nuclear and
    cytoplasmic RNA (Austin & Moyer, 1979).

    Pigs

         FMO purified from hog liver catalyzes NADPH and oxygen-
    dependent sequential S-oxidation of ETU, proceeding through an
    intermediate imidazolinyl sulfenic acid to the corresponding
    sulfinic acid.  Further oxidation to the sulfonic acid was partly
    enzymic and partly due to autooxidation.  The FMO-oxidative pathway
    predominated over P-450 pathways in hog and hamster liver microsomes
    (Poulsen  et al., 1979).

         The mechanism of thyroid peroxidase inhibition by ETU was
    studied  in vitro using purified thyroid peroxidase obtained from
    hog thyroid. ETU inhibited iodination reactions catalysed by thyroid
    peroxidase.  Inhibition occurred only in the presence of iodide ion
    and proceeded with concomitant oxidative metabolism of ETU to
    imidazoline and bisulfite ion.  The inhibition ceased upon
    consumption of ETU, with no loss of enzymatic activity and
    negligible covalent binding of ETU to the enzyme.  This reversible
    thyroid peroxidase inhibition contrasts with the activity of the
    therapeutic antithyroid drugs such as methimazole which act as
    suicide inhibitors via covalent binding to the prosthetic heme group
    (Doerge & Takazawa, 1990).

    Toxicological studies

    Acute toxicity studies

         ETU is slightly toxic after oral administration to mammalian
    species with measured LD50 values ranging from 545 mg/kg bw in
    pregnant rats (Teramoto, 1978) to 4000 mg/kg bw in adult mice
    (Lewerenz & Plass, 1984). The acute toxicity of ETU in various
    animal species is given in Table 1.

    Guinea-pigs

         ETU (purity not stated) is a moderate to weak sensitizer in the
    Hartley strain female guinea-pig by the guinea-pig maximization
    test.  With induction concentrations of 5% (intradermal) or 25%
    (topical) and challenge concentrations of 2% or 0.5% (topical),
    females responded positively at 24 hours (1/10 at 0.5%; 7/10 at 2%)
    but not at the 48-hour reading (0/10 at 0.5%; 0/10 at 2%).  In the
    same studies, cross sensitization responses were also seen with
    maneb, mancozeb and zineb after induction with ETU.  Responses
    ranged from 0-40% (4/10) at 24 hours and from 0-20% (2/10) at 48
    hours.  Induction with ETU followed by challenge with maneb
    generally gave a slightly higher overall response (10-40%) than did
    induction by maneb followed by challenge with ETU (0-20%)
    (Matsushita  et al., 1976).


        Table 1.  Acute toxicity of ETU
                                                                                                      

    Species              Sex                Route        LD50 (mg/kg bw)       References

                                                                                                      

    Mice                 M&F                oral              4000             Lewerenz & Plass, 1984

                          F                 oral            > 3000             Teramoto  et al., 1978b

                 F (9 days pregnant)        oral            > 3000             Khera, 1987 

                         M&F                oral           ca 2400             Peters  et al., 1980b

    Rats                 M&F                oral           ca 2400             Peters  et al., 1980a

                          M                 oral              1832             Graham & Hansen, 1972

                         M&F                oral               940             Lewerenz & Plass, 1984

                F (13 days pregnant)        oral               600             Khera, 1987

                          F                 oral               545             Teramoto et al., 1987b

    Hamsters              F                 oral            > 3000             Teramoto et al., 1987b

                F (11 days pregnant)        oral            > 2400             Khera, 1987

                                                                                                      
    

    Short-term toxicity studies

    Mice

         B6C3F1 mice (10/sex/dose), 8 to 9 weeks of age were fed
    diets containing 0, 125, 250, 500, 1000, or 2000 ppm ETU (97-99%
    purity), equivalent to 0, 19, 38, 75, 150 or 300 mg/kg bw/day for 13
    weeks. Deaths occurred at lower doses but were not dose- or
    compound-related. Body-weight gain and food consumption were
    comparable to controls. Diffuse follicular cell hyperplasia of the
    thyroid occurred at 500 ppm in both sexes (greater than 70%) and was
    statistically significant and dose-related. No effects were observed
    at lower doses. Hepatocellular cytomegaly was also observed at 500
    ppm (4/10 females, 10/10 males) and above. Effects were
    statistically significant and dose-related.  The NOAEL in this study
    was 250 ppm, equivalent to 38 mg/kg bw/day (NTP, 1992).

         Groups of Charles River CD-1 mice (15/sex/dose) were
    administered ETU (100% purity) in the diet at levels of 0, 1, 10,
    100 or 1000 ppm for 3 months, equal to 0, 0.16, 1.7, 18 or 168 mg/kg
    bw/day for males and 0, 0.22, 2.4, 24 or 230 mg/kg bw/day for
    females.  There were no compound-related effects on food
    consumption, body weight, haematology or clinical chemistry
    parameters.  Mixed function oxidase activity was increased in both
    sexes at 1000 ppm, but only statistically significant in males
    (aniline hydroxylase, p-nitroanisole, o-demethylase).  Absolute and
    relative thyroid weights were increased statistically in both sexes
    at 1000 ppm.  Absolute and relative liver weights were significantly
    increased in males at 1000 ppm; relative liver weights were
    significantly increased in females at 100 and 1000 ppm ETU.

         ETU produced thyroid follicular cell hyperplasia and decreased
    colloid density in both sexes at > 100 ppm, with increased
    follicular epithelial cytoplasmic vacuolation and interstitial
    congestion in both sexes at 1000 ppm.  In the liver, ETU produced
    centrilobular hypertrophy, nuclear pleomorphism and increased
    intranuclear inclusions in both sexes at 1000 ppm.  The pigment was
    believed to be similar to lipofuscin.  The NOAEL was 10 ppm ETU,
    equal to 1.7 and 2.4 mg/kg bw/day in males and females, respectively
    (O'Hara & DiDonato, 1985).

    Rats

         Adult male Han:Wistar rats (6/dose group) were given ETU (>
    98% pure) in drinking-water for 28 days.  Drinking-water
    concentrations of ETU were 0, 100, 200 or 300 mg/litre, equal to
    mean daily doses of 0, 11, 18 or 23 mg/kg bw.

         ETU decreased body-weight gain during the exposure. Studies of
    kidney function and morphology indicated that the kidney is not a

    highly sensitive target for ETU-induced toxicity.  ETU did not have
    a permanent physiologically significant effect on urinary sodium,
    potassium, uric acid, protein or glucose excretion, or urinary
    osmolality.  A slight increase in urinary arginine vasopressin (AVP)
    excretion was observed in ETU-treated animals on day 28. No
    prominent light microscopical changes were observed in the kidneys
    of ETU-exposed rats. However, at 300 mg/litre ETU induced clear
    ultrastructural changes in the epithelium of renal proximal tubuli.
    An increased number of lysosomes and myelin figures as well as
    vacuolization and edema were observed in the cytoplasm of the
    epithelial cells of proximal tubules. The proportion of the dose of
    ETU excreted as ETU in urine increased with increasing dose of ETU
    and were 25%, 36% and 49% (Kurrtio  et al., 1991).

         Using an identical protocol as above, a study was conducted to
    determine the effect of ETU on thyroid gland function and
    morphology.  Drinking-water concentrations of ETU were 0, 100, 200
    or 300 mg/litre, equal to mean daily doses of 0, 11, 18 or 23 mg/kg
    bw. Blood samples for T3, T4 and TSH were taken and the levels
    measured using radio immunoassay methods. Thyroid glands were
    extirpated and processed for light and electron microscopy.  ETU
    statistically significantly decreased T4 levels at all doses while
    statistically increasing TSH levels at all doses.  T3 levels were
    also decreased in a dose response manner but values were not
    statistically significant.  There were no ETU-induced morphological
    changes observed under light microscopy. Conspicuous ultra
    structural changes were caused by ETU since a few areas with totally
    destroyed epithelial cells could be found. It was also reported that
    nerves and capillaries might have been affected by ETU (Kurrtio  et
     al., 1986).

         Sprague-Dawley rats (10/sex/group) received 0, 0.63, 1.3, 2.5,
    5.0, or 25 ppm of 98% pure ETU in the diet for 8 weeks.  Twenty-four
    hours after the last feeding, all animals received 5 Ci of 131I
    intraperitoneally. There were no treatment-related effects on
    behaviour, appearance, food intake, organ or body weight or
    macroscopic appearance of organs other than the thyroid.  ETU had no
    clinical chemistry effects at the three low doses.  However at 5 and
    25 ppm slight increases were observed in males and females with
    respect to 131I uptake, protein bound 131I and serum thyroxine.  T3
    uptake power was slightly decreased.  Histopathology of the thyroid
    in treated animals was comparable to control group at all dose
    levels.  The NOAEL in this study was 25 ppm, the highest dose
    tested, equal to 2.6 mg/kg bw/day (Leuschner, 1977).

         F344/N rats 8 to 9 weeks of age (10/sex/group), were fed diets
    containing 0, 60, 125, 250, 500 or 750 ppm of 99% pure ETU for 13
    weeks. All animals survived. Final mean body weights for males were
    decreased at 500 and 750 ppm by 10% and 30%, respectively. Food
    consumption at the same dose levels were decreased 16% and 24%.  

    Final body weights and food consumption of females were decreased at
    750 ppm by 30% and 25%, respectively. Females receiving 60 to 500
    ppm showed a uniform 10% body weight decrease accompanied by food
    consumption decreases of 13% at 250 and 500 ppm.  Histopathology was
    present for the thyroid and pituitary gland of both males and
    females. Diffuse follicular cell hyperplasia of the thyroid was
    present in all animals of both sexes at all doses. In males, focal
    follicular cell hyperplasia and cellular vacuolization of the pars
    distallis of the pituitary gland was statistically significantly
    increased at 250 ppm.  Follicular cell adenomas (3/10) were evident
    at 250 ppm and statistically significant at 750 ppm. Centrilobular
    cytomegaly was observed only at 750 ppm and was statistically
    significant.  In females, follicular cell hyperplasia (4/10) and
    cellular vacuolization of the pars distallis of the pituitary gland
    (10/10) were statistically significant only at 750 ppm.  Follicular
    cell adenomas were observed at 500 and 750 ppm (3/10) but were not
    statistically significant.  Centrilobular cytomegaly of the liver
    was seen only at the high dose and in all animals.  The NOAEL in
    this study was less than 60 ppm, equal to 3.0 mg/kg bw/day for males
    and 4.3 mg/kg bw/day for females, based on histopathological
    findings of diffuse follicular cell hyperplasia in the thyroid (NTP,
    1992).

         In a 90-day study, Sprague-Dawley derived rats (60/sex/dose)
    were fed ETU (96.8% pure) at 1, 5, 25, 125 or 625 ppm.  Controls
    (72/sex) received powdered diet with 1% corn oil.  At 30-day
    intervals (i.e. 30, 60, 90 days) ten rats from each test group were
    sacrificed and serum T3, T4, TBG and TSH concentrations were
    measured. The free thyroxine index (FTI) was also calculated. The
    remaining rats (10/sex/dose/time) were used to determine 125I uptake
    by the thyroid.  Rats receiving 625 ppm ETU showed high mortality
    and marked decrease in body-weight gain. Clinical signs were
    observed at the high dose by day 8 and consisted of excessive,
    salivation, loss of hair, rough and bristly hair coat and scaly skin
    texture.  Necropsy revealed hyperaemia of the thyroids with and
    without enlargement at 125 and 625 ppm for all time intervals. 
    Liver congestion was also evident with dose and time.  Liver changes
    were distinguishable microscopically and appeared to be compound-
    related but not dose-related.  Thyroid to brain weight ratio was
    significantly increased at 125 and 625 ppm at all time periods. 
    125I uptake in the thyroid was statistically significantly decreased
    along with TBG, T3 and T4 values at 125 and 625 ppm. At 25 ppm,
    T4 was statistically significantly decreased only at 60 days. FTI
    was comparable to controls.  Altered thyroid function and increased
    thyroid follicular cell hyperplasia were evident at 125 and 625 ppm. 
    The NOAEL was 25 ppm, equal to 1.7 and 1.9 mg/kg bw/day in males and
    females, respectively (Freudenthal  et al., 1977).

         Osborne-Mendel rats (20 males/group) were fed ETU (purity not
    stated) in the diet at levels of 0, 50, 100, 500 or 750 ppm for 30, 

    60, 90 or 120 days. 131I activity was determined at 4 and 24 hours 
    post-injection (5 Ci) in 20 rats from each group at each sacrifice
    period.

         Body weight was decreased at > 500 ppm throughout the study. 
    Food consumption was reduced at 30 and 90 days at > 100 ppm and
    at 60 and 120 days at > 500 ppm.  Relative thyroid weights were
    increased at 30 days at > 100 ppm, at 90 days at 500 ppm and at
    60/120 days at > 50 ppm.

         Four hours after the injection of 131I, the uptake had
    decreased significantly in rats fed ETU at 500 and 750 ppm at all
    feeding periods.  The uptake of iodine 24 hours after injection was
    decreased significantly in those animals fed ETU at 100, 500 and 750
    ppm.  After the 90-day feeding period, the uptake decreased
    significantly in rats fed the 500 and 750 ppm levels and ranged from
    6 to 13 times lower than control values.

         Histologically there were no differences between the control
    and 50 ppm groups.  At 100 ppm there was slight hyperplasia evident
    in the thyroid gland.  At 500 ppm there was moderate to marked
    hyperplasia, lack of colloid and heightened epithelial walls.  There
    was an increase in vascularization, demonstrating a response to
    increased blood level TSH. At > 500 ppm, an increased incidence
    of follicular adenomas was reported.  One mechanism by which ETU
    acts on the thyroid is via inhibition of iodide peroxidase, which
    oxidizes iodide to iodine (Graham & Hansen, 1972).

    Dogs

         Beagle dogs (2/sex/group) received dietary concentrations of 0,
    200, 980, or 4900 ppm of ETU (98% pure) for 4 weeks.  Body-weight
    gains and food consumption for males were comparable to controls. 
    Intermediate- and high-dose females gained less weight than the
    controls particularly at the high dose.  Haematology results were
    not remarkable.  T3 levels were decreased in high-dose males and
    females as well as mid-dose females.  T4 levels were decreased in
    the mid- and high-dose males and females. Reductions were dose-
    related.  Enlarged thyroids were noted in all animals of the
    intermediate- and high-dose groups.  The NOAEL was 200 ppm, equal to
    6.7-7.4 mg/kg bw/day for males and 7.4-8.5 mg/kg bw/day for females
    (Morgan, 1991).

         Beagle dogs (4/sex/group) received dietary concentrations of 0,
    10, 150 or 2000 ppm of ETU (98% pure with doses corrected to 100%
    active ingredient) for 13 weeks.  Two males in the high dose were
    sacrificed in a moribund state with morbidity attributed to compound
    administration.  All other animals survived to termination. 
    Clinical signs in the high-dose male survivors appeared to be

    unremarkable. All high-dose females showed decreased activity or
    subdued behaviour for various lengths of time (1-5 weeks).  A
    bilobed swelling in the pharyngeal area of two females was also
    reported.  No treatment-related clinical signs were observed in the
    low or intermediate dose groups.  Body-weight changes for survivors
    in treated groups were not statistically significantly different
    when compared to the control group.  However, a slight to severe
    body-weight loss for animals killed moribund was noted.  Food
    consumption was statistically significantly decreased only at the
    high dose for surviving males during weeks 12 and 13, and for
    females during weeks 11 and 12.  Ophthalmological examinations
    showed no remarkable differences between treated and control groups.

         At 13 weeks, males and females of the 150 and 200 ppm groups
    showed statistically significant decreases in haemoglobin, packed
    cell, volume and red blood cell count.  Reticulocyte count was
    statistically significantly increased in females, but not in males. 
    Values for sodium, potassium, and chloride and BUN were all within
    normal limits.  Phosphorous was decreased in males and females at 13
    weeks in the high dose.  The value was statistically significant in
    males. A statistically significant increase in serum protein was
    associated with an increase in serum globulin in the high-dose males
    at 13 weeks.  A statistically significant increase in total
    cholesterol was observed in the intermediate-dose (150 ppm) males at
    weeks 8 and 13 and at weeks 4, 8, and 13 in high-dose (2000 ppm)
    males and females.  An increase in the mean creatinine level was
    noted at weeks 8 and 13 in the high-dose males and females with
    statistical significance attained for males at both time periods.
    ALP was statistically significantly decreased in high-dose males at
    weeks 8 and 13. At week 4, there was a statistically significant
    decrease in mean ASAT in males at the intermediate and high dose and
    in females at all three doses.  A dose response was evident in
    females.  However values at 4 and 8 weeks were comparable to
    controls for all groups of both sexes.  ALAT was statistically
    significantly decreased at week 4 only in females.  Results from
    urinalysis were not remarkable.  Urine colour was however described
    as orange or dark-coloured.  Thyroid hormone assays revealed no
    treatment-related changes in the low- and intermediate-dose groups. 
    However, marked and statistically significant reductions were noted
    for T3 and T4 levels in high-dose animals at weeks 8 and 13.  T4
    was also statistically significantly decreased in males at 4 weeks
    in the high-dose group.

         At week 13, females of the high-dose group showed a marked and
    statistically significant increase in thyroid weights accompanied by
    slight but statistically significant increases in liver and adrenal
    weights. Males of the high-dose group at 13 weeks showed a marked
    increase in thyroid weights concurrent with slight increase in liver
    and adrenal weights.  None of these organ weights were statistically

    significantly increased for males. Macroscopic examinations revealed
    exophthalmia in two males (the survivors) and three females of the
    high-dose group.  Sporadic and slight exophthalmia was also observed
    in one male of the intermediate dose group.  Enlargement of the
    thyroid gland was noted in all surviving high-dose animals as well
    as the two males sacrificed moribund.  The liver and adrenal gland
    both appeared unremarkable as did the remaining tissues examined
    macroscopically. Salient microscopic findings were those of
    hypertrophy of the basophilic cells of the pituitary with micro-
    vacuolisation attended by severe follicular hyperplasia of the
    thyroid gland in all surviving and sacrificed animals of the high-
    dose group.  The liver and adrenal gland were histologically normal. 
    A moderate involution of the thymus of one male and two females of
    the high-dose group was reported.  No treatment-related microscopic
    changes were noted for the low dose (10 ppm) or the intermediate
    dose (150 ppm).  The salient observations related to this study are
    those of the pituitary and thyroid glands of animals receiving the
    highest dose of 2000 ppm (equal to a mean of 66 mg/kg bw/day for
    males and 72 mg/kg bw/day for females). In the pituitary, the lesion
    observed was a hypertrophy of a basophilic cell type with
    microvacuolisation, while in the thyroid, the lesion observed was a
    hyperplasia of the follicular cells with papillary projections of
    the follicular epithelium in the lumen of the follicles.  Similar
    hyperplasia was observed in ectopic nodule of thyroid tissues,
    scattered along the thyroglossal track.  The NOAEL was 10 ppm, equal
    to 0.39 mg/kg bw/day based on decreased haemoglobin, packed cell
    volume and red blood cell count, and increased cholesterol at 150
    ppm.  Effects on the thyroid were found only at 2000 ppm (Briffaux,
    1991).

         Beagle dogs (4/sex/group) received dietary concentrations of 0,
    5, 50 or 500 ppm ETU (expressed as active ingredient taking into
    account the purity index of 98% purity) for 52 weeks. Mortality was
    evidenced in the high-dose group with the death of one male and the
    sacrifice of one male and one female prior to study termination.  No
    treatment-related clinical signs were reported in either the low- or
    mid-dose groups.  Pale mucous membranes in four males and one female
    of the high-dose group was associated with subdued behaviour and a
    change in the colour of the faeces (yellow/orange).  Body weight in
    surviving animals at 52 weeks was decreased 15% in both males and
    females at the high-dose and 8% in the mid-dose males. A dose-
    related decrease was observed in body-weight gain for males of the
    mid- (-43%) and high-dose (-60%) and females of the high-dose (-
    60%).  However, the decreases were not statistically significant. 
    Body-weight gain and body weights in the low-dose group were
    comparable to control group.  There were no statistically
    significant differences in food consumption between treated and
    control groups at 52 weeks.  Food efficiency was generally
    comparable between groups.  Ophthalmological examinations revealed
    comparable findings between all groups.

         Haematological values between control groups and the low- and
    mid-dose groups were comparable.  However, in the high-dose group,
    treatment-related low values (75-80% of normal) in haemoglobin, RBC,
    packed cell volume were reported for all animals dying or sacrificed
    moribund as well as one surviving male.  Additionally the decrease
    in RBC was accompanied by an increased reticulocyte count, a
    decrease in mean corpuscular haemoglobin and an increase in mean
    corpuscular volume.  Low values in platelet count were also observed
    in high-dose animals.  Changes in blood clinical chemistry values
    for sodium, potassium and blood urea nitrogen, cholesterol,
    triglycerides, bilirubin creatinine, gamma glutamyl-transpeptidase
    in surviving animals was not considered treatment-related.  However,
    a slight to moderate increase for total bilirubin was observed for
    animals dying or sacrificed early in the high dose group. Values for
    globulin were statistically significantly higher at weeks 13 and 52
    for the high-dose animals (males and females combined).  A decrease
    in the albumin/globulin ratio was also statistically significant at
    week 52 for the high-dose animals (males and females combined). 
    Elevated values for ASAT and ALAT were reported for both high-dose
    males found dead or sacrificed moribund in the high-dose group. 
    Urinalysis values were unremarkable between groups.

         Thyroid hormone mean values for T4 and T3 were not
    statistically significantly different from control group.  However,
    T3 and T4 values taken shortly prior to death or sacrifice of the
    three high-dose group animals revealed a mean decrease of 50% for
    T3 values and 70% for T4 values.  The values for decedent animals
    were also below the historical range.  Of the two surviving males in
    the high-dose group at 52 weeks, one showed a 47% reduction of T3
    from its pretest level while the other was comparable to its pretest
    level.  T4 values for both high-dose male survivors were generally
    decreased 55% from pretest values.  T3 and T4 values appeared to
    be unaffected in the low-dose and mid-dose groups.  A dose-related
    increase in thyroid weights were observed at week 52 for the
    intermediate and high-dose males and females.  The increase was
    statistically significant for combined males and females for
    absolute, body weight and brain weight ratio (except for brain
    weight ratio in the intermediate dose group).  Necropsy revealed an
    enlargement of the thyroid in one of the two surviving males.

         High-dose animals dying on study or killed in a moribund
    condition all manifested centrolobular hepatocellular necrosis of
    the liver (multifocal and moderately severe in males and multifocal
    and minimal in the female).  Slight pigment accumulation was also
    evident in Kupffer cells.  Hypertrophy of follicular cells with
    dilation of follicles was also seen in the thyroid of one high-dose
    male that was sacrificed.  Pigment accumulations in Kupffer's cells
    and occasionally hepatocytes were observed in both males and females
    of the intermediate and high-dose groups.  Hypertrophy of the

    thyroid with colloid retention was observed in the intermediate and
    high-dose group and ranged in severity from slight to moderately
    severe. The NOAEL was 5 ppm, equal to 0.18 mg/kg bw/day based on
    reduction in body-weight gain, hypertrophy of the thyroid with
    colloid retention, a slight increase in thyroid weight and pigment
    accumulation in the liver at 50 ppm (Briffaux, 1992).

    Monkeys

         Wild-caught rhesus monkeys (5/sex/group) were administered ETU
    (96.8-98.2 purity) in the diet for 5.5 or 6 months at dose levels of
    0, 2, 10, 50, or 250, and 0, 50, 150 or 450 ppm, respectively.

         Results of Study 1:  Body weights were not affected by ETU. 
    Thyroid weight was increased in both sexes at 250 ppm and in females
    at > 50 ppm, resulting from hyperplasia and/or hypertrophy. 
    Females at > 50 ppm also had enlarged pituitary glands.  Ovarian
    weights at 250 ppm were significantly decreased.

         No changes in T3 or TBG were observed.  Serum T4 was
    decreased in both sexes at > 50 ppm identified from FTI analyses. 
    Serum TSH was increased at 250 ppm.  125I uptake also increased at
    > 50 ppm in both sexes.

         Lesions reportedly associated with ETU were identified in the
    pituitary and thyroid gland of animals at > 50 ppm.  These
    included thyroid and pituitary hypertrophy, and thyroid follicular
    cell hyperplasia (moderate to severe). A second study was conducted
    due to the extent of tuberculosis in this first study which
    necessitated the early termination at 5-5.5 months.

         Results of Study 2:  Body weights were not affected by ETU. 
    Thyroid and spleen weights were increased in males at > 150 ppm
    and at all doses in females. Serum T3 decreased in males at
    > 150 pm and in females at 450 ppm.  Serum T4 was decreased in
    both sexes at > 150 ppm.  Radioactive 125I uptake was increased
    in all test groups.  The increased thyroid weight, thyroid iodine
    uptake, decrease in T3, T4 and increase in TSH support the
    evidence for hypothyroidism caused by ETU.

         BUN was elevated in females at 450 ppm along with creatinine
    and a decrease in calcium. Haemoglobin, haematocrit and RBC count
    were decreased in both sexes at 450 ppm.

         Histologic changes were identified in thyroid and pituitary
    glands in both sexes, increasing in severity and incidence with
    increase in dose. Thyroid follicular cell hyperplasia and pituitary
    cytoplasmic vacuolation and swelling were the major changes
    observed.

         The NOAEL for 125I uptake was 10 ppm; the NOAEL for changes in
    T3, T4 and TSH was 50 ppm.  In a separate pathological
    examination, 10 ppm was considered to produce compound-related
    changes in the thyroid gland in 1/7 monkeys.  The NOAEL was
    considered by the authors to be 2 ppm in these combined studies
    (Leber  et al., 1978b).

         However, the monkey studies were considered unreliable because
    one was compromised by ill health of the animals, while little
    reliance could be placed on the effects at the lowest dose used in
    the second.

    Long-term toxicity/carcinogenicity studies

    Mice

         Groups of B6C3F1 mice received perinatal (F0), adult (F1)
    or both exposures to ETU at the following dietary concentrations:
    (F0:F1), 0:0, 0:330, 0:1000, 33-100, 110-330, 330-0, 330-330 or
    330-1000 ppm.  Female C57BL/6N mice were exposed to 0, 33, 110 or
    330 ppm of 99% pure of ETU in feed for one week before breeding, and
    naturally inseminated by C3H/HeN males that received control feed
    only.  ETU exposure continued throughout pregnancy and lactation. 
    Weaning occurred on day 28 post-partum and dietary exposure at these
    same (maternal) concentrations continued until pups were 8 weeks of
    age.  On post-partum day 7, litters were culled to a maximum of 8
    pups, separated by sex after weaning and litter mates co-housed.  At
    8 weeks of age, pups were separated into groups of 60 males and 60
    females to receive dietary concentrations of 0, 330 or 1000 ppm for
    2 years.  Groups of 34 male and 29 female mice that were fed 33 ppm
    of ETU before weaning received 100 ppm for up to 2 years.

         At 9 months, liver weights were increased in groups receiving
    adult exposure concentrations of 330 or 1000 ppm regardless of
    perinatal exposure.  Increases were statistically significant with
    the exception of the 0:330 group.

         Thyroid weights were also reportedly increased in animals given
    1000 ppm.  T4 levels were statistically significantly decreased in
    all animals receiving adult concentrations of 330 or 1000 ppm.  TSH
    levels were statistically significantly increased in males only at
    330:330 and 330:1000 ppm.  Follicular cell vacuolization of the
    thyroid occurred in all animals receiving ETU except those only
    receiving perinatal exposure (i.e. 330:0 ppm).  Animals receiving a
    dose of 33:100 ppm were not reported. Hyperplasia was comparable
    between all groups.

         Hepatocellular adenomas were present in animals receiving 1000
    ppm but were not statistically significant.  Centrilobular 

    cytomegaly was statistically significantly increased in both males
    and females receiving 1000 ppm and in males receiving 110:330 and
    330:330 ppm.  Eosinophilic focus was statistically significantly
    increased in females receiving 1000 ppm.  At 2 years, there was no
    survival disparity between treated and control (0:0 ppm) groups. 
    Clinical signs were not treatment-related.  Body weights of treated
    animals were statistically significantly decreased in both sexes
    compared to 0:0 ppm controls, with the exception of the 330:0 ppm
    dose group which was similar to controls.  Statistically significant
    increases in the number of animals with adenomas or carcinomas were
    observed for hepatocytes, thyroid follicles and posterior pituitary
    in both sexes of high-dose treated, adult only exposed groups when
    compared to 0:0 ppm controls.  At the next lower dose level of 0:330
    ppm both sexes showed a statistically significant increase in
    hepatocellular adenomas or carcinomas.

         Hyperplasia of the thyroid was evident in high-dose males and
    females and in 0:330 ppm females.  Centrilobular cytomegaly was also
    evident in males and females at both dose levels of adult only
    treated animals.  A comparison of animal groups receiving 0:330
    versus 110:330 versus 330:330 ppm showed statistically significant
    increases of thyroid follicular cell hyperplasia (males) and thyroid
    adenomas (females) in perinatal treated groups at 330 ppm.  Similar
    comparisons for hepatocellular neoplasms and pituitary neoplasms
    revealed no statistical differences.  Comparison between animals
    receiving 1000 ppm, with or without perinatal exposure to 330 ppm
    showed no statistical differences for tumours or hyperplasia of the
    thyroid or pituitary.  Treatment of adult females receiving 330 ppm
    with 330 ppm perinatally increased the number of tumours in the pars
    distalis when compared to 0:0 ppm controls as well as those of the
    thyroid.  Animals receiving only a perinatal dose showed a
    comparable response when measured against 0:0 ppm controls.  T4
    levels for both sexes were statistically significantly decreased in
    both sexes at all dose levels. TSH levels were statistically
    significantly increased in animals receiving 1000 ppm (i.e. adult
    and perinatal/adult groups).  Animals receiving 330 ppm during
    adulthood showed elevated TSH levels which were statistically
    significant only in females. Animals receiving perinatal exposure
    only showed TSH values comparable to controls (0:0 ppm) (Chhabra  et
     al., 1992; NTP, 1992).

    Rats

         Charles River rats (60/sex/group) were fed ETU (purity not
    stated) in the diet at levels of 0, 5, 25, 125, 250 or 500 ppm for 2
    years.  Body weights in both sexes were significantly decreased
    initially at doses > 25 ppm; at 500 ppm and above (males) and 125
    ppm and above (females) at 12 months; and at 500 ppm and above (both
    sexes) for the remainder of the study.

         Liver to body-weight ratios were significantly increased at 125
    ppm and through 6 months in males, but comparable to controls for
    the remainder of the study. Relative liver weights in females were
    significantly increased at doses > 125 ppm at 2 months and at
    doses > 250 ppm through 18 months.  No differences between
    control and dose groups were observed at 24 months.  Thyroid to
    body-weight ratio was significantly increased in males at 250 ppm
    and above at 2, 6 and 18 months, and at 125 ppm and above in females
    for the first 12 months.  Thyroid weights were significantly
    increased at 125 ppm and above in males at 12 and 24 months, and at
    250 ppm and above in females at 18 and 24 months.

         Uptake of 131I, expressed as counts/min/mg tissue, was
    significantly decreased in males at 500 ppm throughout the study. 
    Thyroids of females fed 125 ppm and above were hypofunctioning at 6
    months and hyperfunctioning at 12 months.  At 24 months, females had
    a hypofunctioning thyroid at 500 ppm.

         Fewer rats survived to 24 months in the 500 ppm dose group and
    there was also a significant increase in pneumonia which may have
    been further complicated by obstruction of the trachea from enlarged
    thyroids in the animals. Effects in the thyroid were evident at all
    doses.  Increased vacuolarity and hyperplasia in the thyroid were
    evident at 25 ppm and above.  Thyroids of treated rats were
    distinguishable from controls by lobulation, follicular size and
    uniformity, height of follicular epithelium, colloid staining,
    keratinization of follicles, and general size.

         It is possible that ETU initially reduces thyroid activity,
    after which compensation occurs by an increased release of TSH and
    that this increase in TSH stimulated thyroid weight in an attempt to
    overcome the blocking effect of ETU.  The progression to neoplasia
    is believed to be a result of excessive pharmacological stimulation. 
    This is supported, in part, by a lack of thyroid tumours at 1 year
    at 5 or 25 ppm, an increase in tumour incidence after 1 year at 125
    ppm, and confirmed after 2 years in rats fed 250 and 500 ppm. The
    NOAEL in this study was 5 ppm, equivalent to 0.25 mg/kg bw/day
    (Graham  et al., 1973, 1975).

         SPF-Sprague-Dawley (30/sex/dose) received 0, 0.5, 2.5, 5 or 125
    ppm of 96% pure ETU (adjusted to 100%) in feed, 7 days a week for
    either 52 weeks (interim sacrifice of pre-selected animals
    10/sex/dose) or 104 weeks (terminal sacrifice).

         There were no compound-related deaths, clinical signs or
    effects on food consumption. Body-weight gain was slightly impaired
    in males at 125 ppm resulting in group mean body weights 5-6% lower
    than in control males for most of the study.  Female body weight was
    unaffected.  There were no treatment-related changes with respect to
    ophthalmoscopic observations or palpable masses. Haematologic and

    urinalysis finding were comparable to controls. There were no
    treatment-related changes on clinical biochemistry values in males
    receiving 0.5 or 2.5 ppm nor in females receiving 0.5, 2.5 or 5 ppm
    of ETU.  Statistically significant increases were observed in males
    at 125 ppm for total protein, albumin, GGT, cholesterol, bilirubin,
    TSH and T3. T4 and urea values were lower.  T3 values were higher
    at 5 ppm at 29 weeks.  For females at 125 ppm, values for glucose
    and T4 were lower, uric acid, T3 and TSH were higher.  At 125 ppm
    only thyroid weight was higher in males and females at the interim
    and final sacrifice.  Liver weight was slightly higher in both sexes
    but only at the interim sacrifice.  Macroscopic effects were not
    observed at 52 weeks.  However, at 104 weeks the incidence of
    diffuse or modular enlargement of the thyroid gland was increased in
    both sexes at 125 ppm.

         Microscopic examination of the thyroid gland at interim
    sacrifice revealed minimal to moderate diffuse follicular cell
    hyperplasia in 8 animals of the control group and 9, 12, 12 and 20
    animals (both sexes combined) receiving 0.5, 2.5, 5 or 125 ppm of
    ETU, respectively.  The incidence of this finding was significantly
    increased in females at 125 ppm. The severity was increased in males
    at 5 and 125 ppm and in females at 125 ppm.  Slight or moderate
    nodular hyperplasia and follicular adenoma were recorded in 6 and 3
    males of the 125 ppm, respectively.  Minimal or slight focal or
    multifocal cellular hypertrophy of the anterior pituitary was
    recorded in 2 control rats, 2 rats at 2.5 ppm and 7 rats at 125 ppm. 
    Males were predominantly affected.  A significant increase in the
    incidence and severity was calculated for males at 125 ppm.  Other
    morphological alterations observed in treated groups were considered
    secondary to treatment, and affected spleen, thymus gland, auditory
    sebaceous (Zymbal's) glands and lungs.

         At terminal sacrifice, slight to excessive diffuse follicular
    hyperplasia was recorded in 27 animals (sexes combined) receiving
    125 ppm.  The incidence and severity for males and females was
    statistically significant for both sexes.  Slight to severe nodular
    hyperplasia was observed in 9 animals (sexes combined) receiving 125
    ppm.  The incidence and severity of this lesion was significantly
    increased in males.  Follicular adenomas occurred in 1 male of the
    control group and in 4 males given 125 ppm.  Follicular carcinomas
    were observed in 2 males of the high-dose group.  The combined
    incidence of benign and malignant follicular neoplasms yielded a
    clear dose-related trend but did not vary significantly in pairwise
    comparison with the control group.  Anterior pituitary gland
    adenomas were recorded in 8 males and 11 females of the control
    group and 15 males and 10 females at 125 ppm.  There was a clear
    dose-related trend for males and a marginal level of significance by
    pairwise comparison with the control group.  Adenomas of the
    anterior pituitary recorded at the intermediate doses for males and

    females were 5, 8, and 6 and 11, 12, and 11 respectively.  Other
    morphological alterations observed in treated groups were considered
    secondary to treatment and affected the pancreas, lungs and Zymbal's
    glands. The NOAEL was 5 ppm (equal to 0.37 mg/kg bw/day) based on
    changes in clinical chemistry, increased T3, decreased T4,
    increased thyroid and liver weights and an increased incidence and
    severity of diffuse thyroid follicular cell hyperplasia at 125 ppm
    (Schmid  et al., 1992).

         Charles River CD rats (26/sex/group) were fed 0, 175 or 350 ppm
    of technical grade ETU (97% pure) for 2 years.  Follicular or
    papillary carcinomas of the thyroid were observed in 17 males and 8
    females at the high-dose.  At 175 ppm, equivalent to 8.8 mg/kg
    bw/day, 3 males and 3 females had thyroid carcinomas.  Hyperplastic
    goitre was observed in 17 males and 13 females of the high-dose
    group and 9 males and 6 females of the low-dose group.  These
    lesions were not observed in control rats (Ulland  et al., 1972).

         Groups of F344/N rats received perinatal exposure (F0), adult
    exposure (F1) or both to different concentrations (ppm) of ETU as
    follows: F0, F1; 0,0; 0,83; 0,250; 9,25; 30,83; 90,0; 90,83; or
    90,250.  Female rats were exposed to 0, 9, 30 or 90 ppm of 99% pure
    ETU in feed for 1 week before breeding.  All males received control
    feed.  All females were naturally inseminated by males, housed
    individually and continued on their previous diet. ETU exposure
    continued throughout pregnancy and lactation.  Weaning occurred on
    day 28 post partum and dietary exposure at these same concentrations
    continued until the pups were 8 weeks of age.  On post partum day 4,
    litters were culled to a maximum of 8 pups.  Pups were separated by
    sex after weaning and litter mates co-housed (5/cage).  At 8 weeks
    of age, pups were separated into groups of 60 males and 60 females
    to receive adult dietary concentrations of 0, 25, 83 or 250 ppm for
    up to 2 years.

         At 9 months, liver weights were statistically significantly
    increased in males receiving 0,250 or 90,250 ppm of ETU.  Thyroid
    follicular cell hyperplasia was greater than 50% and statistically
    significantly increased for both males and females at the following
    dose levels: 0,83; 0,250; 30,83; 90,83; and 90,250 ppm.  Thyroid
    follicular cell adenomas were observed in both males (3/10) and
    females (1/10) receiving 90,250 ppm.  Values were not statistically
    significant.  T4 values compared to 0,0 ppm controls were
    statistically significantly decreased in all experimental groups of
    both sexes except animals receiving 90,0 ppm.  T3 values were
    statistically decreased in many but not all groups.  The 90,0 ppm
    groups was unaffected.  TSH levels were increased in all dose groups
    and statistically significant only in some female groups.  The 90,0
    ppm groups was only very slightly increased.

         At two years there were no differences in food consumption
    between treated groups and controls with the exception of a decrease
    in the 90,250 ppm group of males during the last month of exposure. 
    Final mean body weights for males and females were comparable to 0,0
    control group with the exception of the 90,250 ppm male dose group
    where the decrease was statistically significant.  Only those
    animals receiving 90,250 ppm showed a statistically significant
    decrease in survival.  Thyroid function values for animals receiving
    90,0 or 0,83 or 9,25 ppm were not statistically different compared
    to controls for males and females at 2 years.  All other doses
    revealed some level of statistical significance in both sexes. 
    There were no clinical findings that could be attributed to thyroid
    dysfunction.  Pathology of the thyroid for animals receiving adult
    only exposures of 0,0; 0,83 or 0,250 ppm revealed statistically
    significant trends and statistically significant increases in high-
    dose males and females for hyperplasia, adenomas, carcinomas and
    adenomas and carcinomas combined.  Animals receiving 0,83 ppm of ETU
    showed statistical increases in hyperplasia (males and females) and
    adenomas (males).  Hyperplasia of the thyroid was the only
    statistically significant effect observed in both sexes when 0,0 and
    90,0 ppm comparisons were made.  Responses between dose groups
    receiving 0,250 or 90,250 ppm revealed a statistically significant
    increase in the number of adenomas in males and carcinomas in both
    males and females.  A comparison of the 0,83; 30,83 and 90,83 dose
    groups in females showed no statistical differences in hyperplasia,
    adenomas, or carcinomas, or adenomas and carcinomas combined.  In
    males only hyperplasia was statistically significantly increased. 
    ETU had no clear effects on the incidences of neoplasms or non-
    neoplastic lesions at sites other than the thyroid gland.  However,
    some groups showed statistically significant increases relative to
    controls in neoplasms of the Zymbal's gland (males and females at
    90,250 ppm) and, mononuclear cell leukaemia (males and females at
    90,250 ppm and males at 90,83 ppm (Chhabra  et al., 1992; NTP,
    1992).

    Rats and Hamsters

         Groups of 20 male and 20 female rats and hamsters were
    administered ETU (purity not stated) in the diet for 24 and 20
    months, respectively, at dose levels of 0, 5, 17, 60 or 200 ppm
    (strain of animals not reported).

         In rats, food consumption was reduced at 60 ppm and above and
    body weight decreased at 17 ppm and above.  Effects on SAP and SGPT
    were not clearly demonstrated due to fluctuations in control levels.
    Cholesterol was increased at 5 ppm in both sexes.  Some hepatic
    enzyme levels were also affected: GPT increased in males at 60 ppm;
    ALP increased at 5 ppm (females) and 17 ppm (males); glucose-6-
    phosphate dehydrogenase did not change.  Thyroid weights were
    significantly increased in both sexes at 60 ppm.  No data were
    available on the histologic examination.

         In hamsters, food consumption and body weight were reduced at
    60 ppm and above.  SAP was increased in both sexes initially, then
    decreased through 18 months.  No effect was observed on SGPT. 
    Cholesterol levels were significantly increased in both sexes at all
    doses compared to controls.  Hepatic enzymes, GPT and ALP, were
    significantly increased in both sexes at all doses.  Glucose-6-
    phosphate dehydrogenase was significantly decreased in both sexes at
    all dose levels.  Relative thyroid weights were significantly
    increased at 200 ppm and above in both sexes.  No data were
    available on the histologic examination (Gak  et al., 1976).

    Reproduction studies

    Rats

         ETU (98% pure) was mixed in the diet and fed to Sprague-Dawley
    rats (25 male and female parents per group) at concentrations of 0,
    2.5, 25 or 125 ppm during a 70-day pre-pairing period and throughout
    pairing, gestation and lactation of 2 generations (one litter per
    generation).  Body weights and mean body-weight gains were reduced
    among the male parents of the F0 generation at 125 ppm.  There were
    otherwise no changes in the viability, clinical appearance or
    behaviour, feed consumption, body weights or weight gain or
    macroscopic appearance of any of the parents, F1 or F2 pups in any
    of the test groups.  Reproduction parameters were unaffected in any
    of the dose groups in either generation.  Histopathologic
    examination indicated compound-related changes in the thyroid and
    anterior pituitary glands at 25 and particularly at 125 ppm in both
    generations.  Thyroid changes in both sexes of both generations
    consisted of follicular cell hypertrophy and hyperplasia which were
    pronounced at 125 ppm and present to a much lesser extent at 25 ppm. 
    Reduced colloid was also present among F1 males and females at 125
    ppm. Adenomas were observed in 3 males (not statistically
    significant).  Pituitary changes consisted of an increased incidence
    and severity of anterior cell hypertrophy in both sexes of both
    generations at 125 ppm, together with a tendency to an increase in
    hypertrophy among parental generation males at 25 ppm and a slight
    increase in cellular vacuolization at 125 ppm.  There was no
    evidence of reproductive organ toxicity up to and including 125 ppm.
    The NOAEL was 2.5 ppm, equal to a range of 0.16-0.38 mg/kg bw/day,
    based on thyroid gland follicular cell hyperplasia and hypertrophy
    at 25 ppm (Dott, 1992).

    Rats/mice

         In the first phase of a two-phase study, adult female rats and
    mice were dosed with ETU (96.7% purity) and then bred to proven male
    sires.  Pregnant females delivered their pups via C-section for
    tissue distribution analyses.  Phase 2 consisted of weanling

    rats/mice dosed for 9 weeks and then analyzed.  Dose levels in the
    diet were: rats: 0, 8, 25, 83 or 250 ppm; mice: 0, 33, 100, 333 or
    1000 ppm (rats: Fischer 344, 3 per group; mice: C57BL/6N, 78 per
    group).  Two weeks after dosing began, breeding was initiated.

         No rat dams or weanlings died.  There was a trend toward
    decreased weight gain in dams in all groups and in weanling males at
    levels > 83 ppm.  Food consumption was also reduced at 250 ppm
    for males only.  No effects on females were observed.  At 250 ppm,
    there was a decrease in pup survival to postnatal day 4.  Thyroid
    hyperplasia was observed in males at all doses and in females above
    8 ppm, increasing in incidence and severity with dose.  Thyroid
    adenomas were reported in males at 83 ppm and above.  Vacuolization
    of pituitary glands in males was noted at 250 ppm.

         There was a significant decrease in body weight in high-dose
    female mice during the period of lactation. Weanling body weights
    were decreased in males and females at 333 ppm and 1000 ppm. 
    Initially, insufficient pregnancies were produced in all dose
    groups. A rebreeding programme, after 6.5 weeks on ETU diets,
    produced sufficient numbers of litters for evaluation.  However, no
    pregnancies were achieved in the high-dose group, and pregnancy rate
    was reduced in other dose groups in comparison to control.  The
    number of pups surviving to day 28 was significantly decreased in
    the high-dose group.  Thyroid hyperplasia and cellular alteration of
    hepatocytes (cytomegaly, karyomegaly) were observed in both sexes at
    1000 ppm.  One male mouse at 333 ppm also had adverse effects in the
    liver (Peters  et al., 1982).

    Special studies on embryotoxicity/teratogenicity

    Rats

         Pregnant Charles River Rats (ChR-CD, Sprague-Dawley) were
    administered 0, 25 or 50 mg/kg bw/day of ETU (98% pure) in DMSO, or
    DMSO alone (vehicle control) or water alone onto the shaved back of
    each animal for 48 hours on days 10 and 11 or days 12 and 13 of
    gestation.

         Maternal body-weight change during the 48-hour administration
    period ranged from +4 to -5%.  Fetuses examined from dams
    administered ETU at 50 mg/kg bw/day on days 10 and 11, showed short
    tails (3/83) and fused ribs (2/83).  However, dams given 50 mg/kg
    bw/day on days 12 and 13 produced fetal deformities in all
    offspring.  Fetal defects were characterized by encephalocele, a
    part or the entire tail missing, missing leg bones, hunchback
    curvature to the spine, short mandible, fusion of ribs and fusion of
    sternebrae.

         A dose of 25 mg/kg bw/day administered only on days 10 and 11
    of gestation did not result in any fetal abnormalities. A dose of 25
    mg/kg bw/day was not administered on days 12 and 13 of gestation
    (Stula & Krauss, 1977).

         ETU (100% purity) was administered orally at doses of 0, 5, 10,
    20, 40 or 80 mg/kg bw/day in distilled water to nulliparous rats
    (Wistar) (10-17 pregnant dams per dose).  Treatment was made for 21-
    42 days before conception to pregnancy day 15, and on days 6-15 or
    6-20 of pregnancy.  Doses of 40 mg/kg bw/day were not toxic to rats;
    however, 80 mg/kg bw/day was lethal to 9 of 11 female rats.  Mean
    fetal weight was reduced at 40 mg/kg bw/day. Measurements of
    sterility, pre-implantation loss and post-implantation survival were
    comparable to controls.  The brain was the most commonly affected
    organ.  ETU induced meningoencephalocele, meningorrhagia,
    meningorrhea, hydrocephalus, obliterated neural canal, abnormal
    pelvic limb posture with equinovarus, and short or kinky tail at 10
    mg/kg bw/day in all phases of the study.  Although no abnormalities
    were reported in rats at 5 mg/kg bw/day, there was a higher
    frequency of delayed ossification of the parietal bone, compared to
    controls.  The NOAEL for embryo/fetotoxicity was 5 mg/kg bw/day
    based on teratogenic effects observed at 10 mg/kg bw/day.  The NOAEL
    for maternal toxicity was 40 mg/kg bw/day (Khera, 1973).

         ETU was given by gavage (distilled water, 5 ml/kg bw/day) on
    days 6-20 of gestation to pregnant Sprague-Dawley rats (22/group) at
    doses of 0, 15, 25, or 35 mg/kg bw/day, and dams were sacrificed on
    day 21 for examination of uterine contents.  Maternal appearance,
    behaviour, and body-weight gain were generally unaffected, and the
    incidence of pregnancy was comparable among the groups. No adverse
    effect was noted on the average numbers of implantations, live
    fetuses, or percentages of resorption sites per litter.  Mean fetal
    body weights were decreased in a dose-related manner, but were only
    significantly reduced at 35 mg/kg bw/day (13-15% lower).  ETU at 35
    mg/kg bw/day produced external malformations including cranial
    meningocele and meningorrhea, severe hindlimb talipes, and a non-
    significant incidence of hydrocephaly.  Short and/or kinky tails
    were noted in 43.5% of the fetuses.  Soft tissue examinations
    revealed higher incidences of dilated brain ventricles at 25 and 35
    mg/kg bw/day (33.5 and 93% of the fetuses, respectively) and of
    hydroureter and dilated ureter at 35 mg/kg bw/day, and skeletal
    examinations revealed a reduced ossification of skull bones and a
    significantly increased incidence of dumbbell-shaped or bilobed
    vertebral centra (33.5% of fetuses).  There were no other treatment-
    related increases in skeletal variants among any of the experimental
    groups, and no treatment-related effects of any kind identified in
    the 15 mg/kg bw/day group. The NOAEL for maternal toxicity was 35
    mg/kg bw/day.  The NOAEL for embryo/toxicity and teratogenicity was
    15 mg/kg bw/day based on higher incidences of dilated brain
    ventricles at 25 mg/kg bw/day (Saillenfait  et al., 1991).

         ETU (100% purity) was administered via oral gavage at 40 mg/kg
    bw/day from days 7 to 15 of gestation to pregnant CR rats (10-12
    rats/group). Rats were hypothyroid and euthyroid.  There was a
    problem, however, in maintaining the euthyroid state in rats given
    supplement.  Rats were also given thyroxine to determine if ETU
    teratogenicity occurred through alterations of maternal thyroid
    function.  ETU was found to be teratogenic in the rat but not
    through alteration of maternal thyroid status.  It was also
    demonstrated that ETU lowered serum T4; that hypothyroidism  per se
    increased the background level of malformations in the rat; that T4
    alone was embryotoxic but not teratogenic; and that hypothyroidism
    altered the spectrum of malformations in response to ETU both
    quantitatively and qualitatively (Lu & Staples, 1978).

         Virgin Sprague-Dawley rats were mated one-to-two with males
    and, after pregnancy was verified, were administered ETU (unknown
    purity), T3/T4 and sodium iodide via oral gavage in varying
    concentrations, either singly or in combination, as well as a
    control solution of water only, from day 7 to day 20 of gestation.
    Dosing regimen was as follows:

    Dose group                                    Total rats per
                                                    group

    Control 1 ml distilled water                    14
    T3 20 g/kg bw/day + T4 100 g/kg bw/day        10
    Sodium iodide 333 g/kg bw/day                  10
    ETU 20 mg/kg bw/day                             10
    ETU 20 mg/kg bw/day + sodium iodide             16
    ETU 20 mg/kg bw/day + T3/T4                     16
    ETU 40 mg/kg bw/day                             11
    ETU 40 mg/kg bw/day + sodium iodide             14
    ETU 40 mg/kg bw/day + T3/T4                     15

         Each pregnant dam was killed on day 20 by chloroform
    asphyxiation and the fetuses removed via hysterotomy.  The number of
    resorptions, live/dead fetuses and fetal birth weights were
    determined.  Skeletal analyses were performed on 1/3 and visceral
    analyses on 2/3 of the fetuses. Results indicated a possible
    reduction in the teratogenic response to ETU for some malformations
    when T3/T4 was administered in conjunction with ETU.  For example,
    20 and 40 mg/kg bw/day ETU (alone) produced 97.6 and 94.5% incidence
    of hydrocephaly, respectively.  In combination with T3/T4 these
    same levels produced 19.6 and 74.5% incidence, respectively. These
    results indicate that the teratogenic potential of ETU may in part
    be secondary to the thyroid toxicity of ETU (Emmerling, 1978b).

    Rats, mice and hamsters

         Wistar-Imamichi rats, JCL-ICR mice, and Syrian golden hamsters
    10 weeks or older were mated overnight and examined the next morning
    for the presence of a vaginal plug or spermatozoa in vaginal smears. 
    Evidence of copulation was designated as day zero of gestation. 
    Pregnant females were given daily oral doses of ETU by gavage during
    the period of organogenesis.  Doses given to rats, mice and hamsters
    were, respectively 0, 10, 20, 30, 40 or 50 mg/kg bw/day, 0, 200, 400
    or 800 mg/kg bw/day and 0, 90, 270, or 810 mg/kg bw/day.  Rats,
    mice, and hamsters were sacrificed on days 20, 18, and 14,
    respectively.

         Dams did not show signs of toxicity and none died in any
    species. There were no statistically significant differences between
    treated and control rats for the mean number of implants and live
    fetuses reported.  Mean fetal weight for both males and females was
    statistically significantly decreased at 30 mg/kg bw/day and higher.
    The percent of fetal death was also statistically significantly
    increased at 50 mg/kg bw/day.  Mice showed no statistically
    significant changes between treated and control values for any of
    the prenatal developmental parameters (i.e mean number of implants,
    mean number of live fetuses, percent fetal death and mean fetal body
    weight).  The mean number of implants for hamsters were comparable
    to control values for all doses. There was a statistically
    significant decrease in the mean number of live fetuses born
    attendant to a decrease in mean fetal body weight for males and
    females which was statistically significant at 810 mg/kg bw/day. 
    Mean fetal body weight for females was also statistically
    significantly lower at 270 mg/kg bw/day without other prenatal
    developmental effects. Gross external anomalies were not meaningful
    between controls and treated groups for mice. Rats manifested a
    short or kinky tail at 30 mg/kg and above in 80% of the offspring.

         In rats, meningocele was observed in 66% of the offspring at 40
    mg/kg bw/day and above and micrognathia in 30% of the offspring at
    50 mg/kg bw/day.  Hamsters showed multiple type anomalies at 810
    mg/kg bw/day and included such signs as cleft palate, short or kinky
    tail, and oligodactyly.  Short or kinky tail was observed in 2% of
    the animals at 270 mg/kg bw/day and 42% of the animals at 810 mg/kg
    bw/day. The LOAEL in rats was 30 mg/kg and seen as curved clavicles. 
    There was an increased incidence of curved clavicles, fused/wavy
    ribs, fused sternebrae, malformed vertebrae and scoliosis at 40 and
    50 mg/kg bw/day. The LOAEL was 90 mg/kg bw/day in hamsters based on
    a 2% incidence of malformed lumbar and sacral vertebrae, a 4%
    incidence at 270 mg/kg bw/day and a 51% incidence at 810 mg/kg.
    There were no brain or visceral anomalies observed in mice. 
    Dilation of the lateral 4th ventricle was observed in 5% of hamsters
    at the high dose, none at lower doses and 2% in controls.  Dilation
    of the lateral or 4th ventricle in rats was observed in 2% and 39%

    of rats at doses of 10 and 20 mg/ kg bw/day.  At 30 mg/kg bw/day and
    above the response was 100%. No maternal toxicity occurred at the
    doses tested.  The NOAEL for embryo/ fetotoxicity in the rat was 10
    mg/kg bw/day based on dilation of the lateral or fourth ventricle at
    20 mg/kg bw/day.  The NOAEL for embryo/fetotoxicity in the hamster
    was 90 mg/kg bw/day based on decrease of fetal body weight at 270
    mg/kg bw/day.  The NOAEL for mice was greater than 80 mg/kg bw/day
    (Teramoto  et al., 1978).

    Hamsters

         ETU (purity > 99%) was administered orally to pregnant Syrian
    hamsters at doses of 600, 1200, 1800 or 2400 mg/kg bw on day 11 of
    gestation.  All dams were killed on day 15 of gestation for necropsy
    and fetal examination.  There were only 5 pregnant dams in the
    control group compared to 8-10 in the treated groups.

         Maternal toxicity was not reported at any dose.  However, there
    was an increased incidence of resorbed fetuses and fetuses dying
    late in gestation with an associated decrease in the number of live
    fetuses at 2400 mg/kg bw. Fetal body weights were similarly reduced
    at 1800 mg/kg bw.  Malformations were evident at > 1200 mg/kg bw,
    with no adverse effect reported at 600 mg/kg bw.  Fetal anomalies
    included cleft palates ectrodactyly, hydrocephalus and hypoplastic
    cerebellum.  There was also an increased incidence of delayed
    ossification of the calcarium and sternebrae defects.  As with other
    species (i.e. rat, cat), the brain was particularly sensitive to
    ETU, although at higher dose levels (Khera  et al., 1983).

    Mice/rats/hamsters/guinea-pigs

         Time-pregnant random-bred CD-1 mice, Sprague-Dawley rats golden
    hamsters and Hartley strain guinea-pigs were used.  Maneb (80%
    pure), ETU (melting point 197-198 C) and EBIS were administered by
    gastric intubation.  Control animals received vehicle alone (water
    or corn oil) or remained untreated.  Prenatal studies were conducted
    on rats given maneb (480, 240, 120, 0 mg/kg bw/day for days 7-16) or
    ETU (80, 40, 30, 20, 10, 5, 0 mg/kg bw/day for days 7-21) or EBIS
    (30, 25, 7.5, 0 mg/kg/ bw/day for days 7-21).  Prenatal studies were
    also conducted on mice given maneb (1500, 750, 375, 0 mg/kg bw/day
    for days 7-16) or ETU (200, 100, 0 mg/kg bw/day for days 7-16) or
    EBIS (200, 100, 50, 0 mg/kg bw/day for days 7-16).  Prenatal studies
    were also conducted with hamsters and guinea-pigs but only with ETU. 
    Hamsters received either 100, 50, 25 or 0 mg/kg of ETU on days 5-10
    of gestation. Guinea-pigs received 100, 50 or 0 mg/kg bw/day of ETU
    on days 7-25.  Post-natal studies were only conducted on rats.  Rats
    received either maneb (480, 240, 0 mg/kg bw/day) or ETU (30, 25, 20,
    0 mg/kg bw/day) or EBIS (30, 15, 0 mg/kg bw/day) on days 7-15 of
    gestation.  Mice were killed on day 18, rats on day 21, hamsters on

    day 15 and guinea-pigs on day 35.  Animals designated for post-natal
    study were allowed to litter normally.  Litters were normalized to 4
    individuals of each sex and weaned on day 22 post-partum.

         In toxicity studies conducted to determine dose levels, all
    compounds tested proved to be more toxic in the rat than the mouse. 
    Hind limb paralysis was observed in rats given maneb at the high
    dose of 600 mg/kg bw/day.  No toxicity was noted in mice given a
    dose of 1500 mg/kg bw/day of maneb.  EBIS produced death in rats at
    75 mg/kg bw/day and hind limb paralysis at 50 mg/kg bw/day. 
    Decreased body-weight gain and death was observed in mice given 100
    mg/kg bw/day.  ETU produced lethality in rats at the high dose of 80
    mg/kg bw/day.  ETU was not toxic in mice (300 mg/kg bw/day) hamsters
    (150 mg/kg bw/day) or guinea-pigs (100 mg/kg bw/day) at the high
    dose tested.

         Maneb administered to pregnant rats resulted in significant
    dose-related decrease in maternal weight gain and increase in liver
    to body-weight ratios.  Fetal weight and caudal ossification were
    significantly reduced only at the highest dose tested (480 mg/kg
    bw/day). At the high dose (480 mg/kg bw/day), 18 fetuses from 4
    litters manifested hydrocephalus.  Maternal mice given maneb
    manifested increased liver/body-weight ratios and decreased caudal
    ossification beginning at 375 mg/kg.  Dose-related trends were
    evident.  EBIS given to rats and mice did not result in adverse
    fetal effects.  Average maternal weight gain was decreased in rats
    at 30 mg/kg bw/day.  Maternal weight gain in mice was not affected. 
    Average liver to body-weight ratio was increased at the high dose
    tested for rats (30 mg/kg bw/day) and mice (200 mg/kg bw/day).  ETU
    administered to rats at the high dose of 80 mg/kg bw/day resulted in
    25% maternal mortality and reduced weight gain.  Fetal toxicity was
    also observed at 80 mg/kg bw/day and included mortality, decreased
    weight, decreased ossification and edema.  Gross defects of the
    skeletal system and central nervous system were noted in a majority
    of fetuses. At 40 mg/kg bw/day, fetal weight and ossification were
    reduced and hydrocephalus and encephalocele were evident.
    Hydrocephalus was seen at 20 mg/kg bw/day and decreased fetal body
    weight at 10 mg/kg bw/day. ETU produced an increase in the liver-
    body-weight ratio of mice at 100 mg/kg bw/day and 200 mg/kg bw/day
    and an increase in the number of supernumerary ribs at 200 mg/kg
    bw/day. No apparent effects were observed in hamsters or guinea-
    pigs. Post-natal studies with maneb resulted in delayed eye opening
    in males.  Post-natal observations with EBIS resulted in decreased
    fetal body weight at day 22 in females only as well as delayed eye
    opening.  ETU administration produced no observable post-natal
    effects with the exception of increased total open field activity in
    males.  There were no apparent differences reported in open field
    activity between male fetuses surviving the high dose (30 mg/kg
    bw/day) with hydrocephalus and their apparently normal mates. 
    Hydrocephaly was not observed at lower doses (Chernoff  et al.,
    1979).

    Rabbits

         ETU (100% purity) was administered orally at doses of 0, 5, 10,
    20, 40 or 80 mg/kg bw/day in distilled water to nulliparous rabbits
    (New Zeeland white).  There were 5-7 pregnant does per group. 
    Treatment was made from days 7 to 20 of pregnancy.  No toxicity was
    apparent in rabbits given 80 mg/kg bw/day.  Fetal weights were not
    affected. Measurements of sterility, pre-implantation loss and post-
    implantation survival were comparable to controls. Rabbits presented
    no evidence of malformations at the doses administered.  However,
    there was an increase in resorption sites, decreased brain weight,
    and degeneration of the proximal convoluted tubules in the kidneys
    of fetuses at 80 mg/kg bw/day.  The NOAEL for maternal toxicity was
    80 mg/kg bw/day and for embryo/fetotoxicity the NOAEL was 40 mg/kg
    bw/day (Khera, 1973).

    Cats

         ETU (purity not stated) was administered orally (in gelatin
    capsules) to pregnant European and Persian breed cats (7-14 cats per
    group) at doses of 0, 5, 10, 30 or 60 mg/kg bw/day on days 16-35 of
    gestation or 120 mg/kg bw/day from days 16 to 34 of gestation.  No
    effect was evident at 5 mg/kg bw/day.  However, at > 10 mg/kg
    bw/day decreased ataxia, tremors and hindlimb paralysis were
    observed. No pregnant cats survived in the 30 and 60 mg/kg bw/day
    dose groups. The remaining cats showed no apparent treatment-related
    effect on fetal viability or fetal weight. Although this study was
    inconclusive in many respects, there was an increased incidence of
    toxicity to the central nervous system at 10 mg/kg bw/day.  Further,
    at 5 and 120 mg/kg bw/day, there were anomalous fetuses in each
    group.  Incidences of exencephaly, hydrocephaly, cleft palate,
    kyphoscoliosis, umbilical hernia, coloboma, and spina bifida were
    observed in these two treated groups.  Similar anomalies were
    observed in the rat (Khera & Iverson, 1978).

    Special studies on genotoxicity

         ETU has been the subject of many  in vitro and  in vivo
    studies for genotoxicity.  It induced mutations in bacteria at very
    high doses but variable responses have been obtained in other types
    of mutation assays.  Acceptable assays for other genotoxicity
    endpoints  in vitro were generally negative, while all  in vivo
    assays were negative.  The Meeting concluded that ETU was not
    genotoxic.  The results of genotoxicity assays on ETU are given in
    Table 2.


        Table 2.  Results of genotoxicity assays on ethylenethiourea
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    1. GENE MUTATION ASSAYS

    1.A. Bacterial Gene Mutation Assays 

    Salmonella            S. typhimurium                   10-20 000 g/plate;              ?           Positive            Teramoto et al., 
    reversion             TA1535, TA1536,                  in DMSO?                                     (TA1535 without     1977; Shirasu 
    assay                 TA1537, TA1538, G46                                                           activation)         et al., 1977

                          S. typhimurium                   0.2-2000 g/plate               >98%         Negative            Brooks & Dean, 1981
                          TA1535, TA1537, 
                          TA1538, TA92, TA98, 
                          TA100

                          S. typhimurium                   5-5000 g/plate;                >98%         Negative            Richold & Jones,
                          TA1535, TA1537,                  in DMSO                                                          1981
                          TA1538, TA98, TA100

                          S. typhimurium                   0.1-2000 g/plate;              >98%         Negative            Rowland & Severn, 
                          TA1535, TA1537,                  in DMSO                                                          1981
                          TA1538, TA98, TA100

                          S. typhimurium                   10-5000 g/plate;               >98%         Positive            Simmon & Shepherd,
                          TA1535, TA1537,                  in DMSO                                      (TA1535 with        1981
                          TA1538, TA98, TA100                                                           & without 
                                                                                                        activation)

                          S. typhimurium                   4-2500 g/plate                 >98%         Negative            Trueman, 1981
                          TA1535, TA1537, 
                          TA1538, TA98, TA100

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    Salmonella            S. typhimurium                   1000-20 000 g/plate; no         ?           Positive            Autio et al., 1982
    reversion             TA1950                           activation used; in DMSO
    assay (cont'd)

                          S. typhimurium                   >500 g/plate;                   ?           Positive            Moriya et al., 1983
                          TA1535, TA1537,                  in DMSO?                                     (TA1535 with 
                          TA1538, TA98, TA100                                                           and without 
                                                                                                        activation)

                          S. typhimurium                   100-10 000 g/plate;            98.4%        Positive            Mortelmans et al.,
                          TA1535, TA1537, TA98,            in DMSO                                      (TA1535 with        1986 (SRI)
                          TA100                                                                         & without 
                                                                                                        activation)

    Salmonella            S. typhimurium                   10-1000 g/ml;                  >98%         Negative            Gatehouse, 1981
    reversion assay;      TA1535, TA1537, TA98             in dimethylacetamide
    fluctuation test

    Salmonella            S. typhimurium                   200-80 000                       ?           Positive            Schupbach & Hummler,
    forward               TA1530                           g/plate                                                         1977
    mutation assay

    E. coli               E. coli WP2 hcr                  10-10 000 g/plate               ?           Negative            Teramoto et al., 
    reversion assay                                                                                                         1977; Shirasu 
                                                                                                                            et al., 1977

                          E. coli 343/113/uvrB             200-4000 g/ml;                 >98%         Positive            Mohn et al., 1981
                                                           in phosphate buffer                          (galR- & arg+
                                                                                                        systems with 
                                                                                                        activation)

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    E. coli               E. coli WP2 uvrA                 10-1000 g/ml;                  >98%         Negative            Gatehouse, 1981
    reversion assay;                                       in dimethylacetamide
    fluctuation test

    Host mediated         S. typhimurium                   500-6000 mg/kg;                  ?           Negative            Schupbach & Hummler,
    assay Swiss           G46                              in DMSO                                                          1977
    albino mouse

    Swiss albino          S. typhimurium                   670-6000 mg/kg;                  ?           Weak                Schupbach & Hummler,
    mouse                 TA1530                           in DMSO                                      Positive            1977

    Male JCL-SD rat       S. typhimurium                   200-400 mg/kg                    ?           Negative            Teramoto et al.,
                          G46                                                                                               1977; Shirasu 
                                                                                                                            et al., 1977

    Male JCL-ICR          S. typhimurium                   200-400 mg/kg                    ?           Negative            Teramoto et al.,
    mouse                 G46                                                                                               1977; Shirasu 
                                                                                                                            et al., 1977

    1.B. In Vitro Mammalian Gene Mutation Assays 

    Mammalian gene        Mouse lymphoma                   140-3000 g/ml;                 >98%         Negative            Jotz & Mitchell, 
    mutation assay        L5178Y TK+/-                     in DMSO                                                          1981

                          Mouse lymphoma                   25-3600 g/ml;                  NTP          Positive            McGregor et al., 
                          L5178Y TK +/-                    in DMSO                         chemical                         1988
                                                                                           repository

                          Chinese hamster ovary            1000-2000 g/ml;                >98%         Negative            Carver et al., 1981
                          (CHO-AT3-2; several loci)        in DMSO

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    1.C. In Vivo Gene Mutation Assays

    Sex-linked            D. melanogaster                  0.25-2.5%;                       ?           Negative            Mollet, 1975
    recessive lethal                                       in sugar water
    assay

                          D. melanogaster                  250 ppm; in DMSO                >98%         Negative            Valencia & 
                                                                                                                            Houtchens, 1981

                          D. melanogaster                  4900 ppm injection;             97%          Inconclusive        Woodruff et al., 
                                                           12 500 ppm feed; in water                                        1985 Mason et al., 
                                                                                                                            1992

                          D. melanogaster                  5100 ppm                        98.4%        Inconclusive        Mason et al., 1992

    1.D. Yeast and Other Fungal Assays 

    Forward mutation      S. pombe                         0.1-1 g/ml;                    >98%         Negative            Loprieno, 1981
                                                           in DMSO

                          A. nidulans                      0.22-116 mM;                    >98%         Negative            Crebelli et al., 
                                                           no activation used;                                              1986
                                                           in DMSO

    Reverse mutation      S. cerevisiae XV185-14C          88.9-889 g/ml;                 >98%         Equivocal           Mehta & von Borstel,
                                                           in DMSO                                                          1981

    1.E. Plant Test

    Mutation              Tradescantia                     9.79 X 10-5 M;                   ?           Positive            van't Hof & 
                          clone 4430                       in DMSO?                                                         Schairer, 1982

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    2. STRUCTURAL CHROMOSOMAL ALTERATIONS

    2.A. In Vivo Chromosomal Alterations in Mammalian Cells

    In vitro              Chinese hamster cell line        1000-3200 g/ml                  ?           Negative            Teramoto et al., 
    chromosomal           (Don)                                                                                             1977; Shirasu 
    aberrations                                                                                                             et al., 1977

                          Chinese hamster ovary            1670-5000 g/ml;                >98%         Positive            Natarajan & van 
                          (CHO) cells                      in DMSO                                      (with and           Kesteren-van 
                                                                                                        without             Leeuwen, 1981
                                                                                                        activation)

                          Chinese hamster ovary            6000-10 000 g/ml;              >98%         Negative            NTP, 1992
                          (CHO) cells                      in DMSO

    2.B. In Vivo Chromosomal Alterations

    Bone marrow           Male & female Wistar rat         50-400 mg/kg;                    ?           Negative            Teramoto et al., 
    cytogenetics                                           in aqueous soln                                                  1977; Shirasu 
                                                                                                                            et al., 1977

    Micronucleus assay    Female ICR mouse                 150-450 mg/kg;                   ?           Negative            Seiler, 1975
                                                           in DMSO

                          Male & female Swiss albino       700-6000 mg/kg;                  ?           Negative            Schupbach & Hummler,
                          mouse                            in gummi arabicum                                                1977

                          Male ICR mouse                   220-880 mg/kg;                  >98%         Negative            Kirkhart, 1981
                                                           in DMSO

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    Micronucleus assay    B6C3F1 mouse                     880-1416 mg/kg;                 >98%         Negative            Salamone et al., 
    (cont'd)                                               in DMSO                                                          1981

                          Male & female CD-1 mouse         220-880 mg/kg;                  >98%         Negative            Tsuchimoto & Matter,
                                                           in DMSO                                                          1981

    Dominant lethal       Swiss albino mouse               500-3500 mg/kg;                  ?           Negative            Schupbach & Hummler,
    assay                                                  in gummi arabicum                                                1977

                          JCL-ICR mouse                    300-600 mg/kg;                   ?           Negative            Teramoto et al.,
                                                           in water with gum arabic                                         1977; Shirasu 
                                                                                                                            et al., 1977

                          C3H/HeCr mouse                   150 mg/kg;                       ?           Negative            Teramoto et al., 
                                                           in gum arabic soln                                               1978

    Reciprocal            D. melanogaster                  500 ppm;                        >98%         Negative            NTP, 1992
    translocations                                         in 5% sucrose soln

    3. OTHER GENOTOXIC EFFECTS

    3.A. DNA Damage and/or Repair Assays and Related Tests

    Rec assay             B. subtilis H17, M45             20-4000 g/disk                  ?           Negative            Teramoto et al.,
                                                                                                                            1977; Shirasu 
                                                                                                                            et al., 1977

                          B. subtilis H17, M45 spores      2000 g/disk;                   >98%         Positive            Kada, 1981
                                                           in DMSO                                      (without 
                                                                                                        activation)

                          E. coli WP2, WP67, CM871         Not specified                   >98%         Negative            Green, 1981

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    Rec assay (cont'd)    E. coli WP2, WP67, CM871         Not specified                   >98%         Negative            Tweats, 1981

                          E. coli several deficient        500 g/ml; in DMSO              >98%         Positive            Ichinotsubo et al.,
                          strains                                                                       (with activation)   1981

                          S. typhimurium TA1538,           125-2000 g/disk;                ?           Negative            Rashid & Mumma, 1986
                          TA1978;                          in DMSO
                          E. coli K12 & WP2

    Inhibition of DNA     E. coli polA                     2273 g/ml                      >98%         Positive            Rosenkranz et al.,
    polymerase I                                                                                        (without            1981
                                                                                                        activation)

    Lambda prophage       E. coli (lysogenic)              2-20 mg/ml; only activation     >98%         Positive            Thomson, 1981
    induction                                              used; in DMSO

    E. coli               E. coli PQ37                     Not specified; in DMSO           ?           Negative            Quillardet et al.,
    SOS Chromotest                                                                                                          1985

    In vitro              Human fibroblasts from           Not specified; in DMSO          >98%         Negative            Agrelo & Amos, 1981
    unscheduled DNA       skin biopsies
    synthesis (UDS)

                          HeLa S3 human cells              Not specified; in DMSO          >98%         Inconclusive        Martin & McDermid,
                                                                                                                            1981

                          WI-38 human fibroblasts          63-2000 g/ml; in DMSO          >98%         Negative            Robinson & Mitchell,
                                                                                                                            1981

    In vitro unschuled    Rat hepatocytes                  9.6 X 10-9 - 3.2 X 10-3 M        ?           Negative            Althaus et al.,
    DNA synthesis         (nuclei isolation)                                                                                1982
    (UDS)

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    In vivo/in vitro      Female B6C3F1 mouse              1500 mg/kg; in corn oil         98%          Negative (UDS);     Frank & Muller, 1988
    unscheduled DNA       hepatocytes                                                                   Positive (S-
    synthesis (UDS)/S                                                                                   phase increase)
    phase analysis

    3.B. Sister Chromatid Exchange (SCE) Assays

    In vitro              Chinese hamster ovary            25-1000 g/ml;                  >98%         Negative            Evans & Mitchell,
    SCE assays            (CHO) cells                      in DMSO                                                          1981

                          Chinese hamster ovary            1670-5000 g/ml;                >98%         Negative            Natarajan & van
                          (CHO) cells                      in DMSO                                                          Kesteren-van 
                                                                                                                            Leeuwan, 1981

                          Chinese hamster ovary            0.01-100 g/ml                  >98%         Negative            Perry & Thomson, 
                          (CHO) cells                                                                                       1981

                          Chinese hamster ovary            500-10 000 g/ml;               >98%         Negative            NTP, 1992
                          (CHO) cells                      in DMSO

    In vivo SCE assays    Male CBA/J mouse bone            1000 mg/kg; in DMSO             >98%         Negative            Paika et al., 1981
                          marrow and liver

    3.C. Yeast and other Fungal Assays

    Mitotic aneuploidy    S. cerevisiae D6                 500 g/ml; in DMSO              >98%         Positive            Parry & Sharp, 1981

    Chromosome            A. nidulans                      19.6-78.3 mM; no activation     >98%         Positive            Crebelli et al., 
    malsegregation                                         used; in DMSO                                                    1986

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    Mitotic gene          S. cerevisiae D4                 33-333.33 g/plate;             >98%         Negative            Jagannath et al., 
    conversion                                             in DMSO                                                          1981

                          S. cerevisiae JD1                50 g/ml; in DMSO               >98%         Positive            Sharp & Parry, 1981a
                                                                                                        (without 
                                                                                                        activation)

                          S. cerevisiae D7                 2000-4000 g/ml                 >98%         Negative            Zimmermann & Scheel,
                                                                                                                            1981

    Mitotic               S. cerevisiae T1, T2             1000 g/ml; in DMSO             >98%         Negative            Kassinova et al.,
    crossing-over                                                                                                           1981

                          A. nidulans                      19.6-78.3 mM; no activation     >98%         Negative            Crebelli et al., 
                                                           used; in DMSO                                                    1986

    Intrachromosomal      S. cerevisiae RS112              5-40 mg/ml; no activation used   ?           Positive            Schiestl et al., 
    recombination                                                                                                           1989

    Differential          S. cerevisiae, T5                Not specified;                  >98%         Negative            Kassinova et al., 
    killing                                                in DMSO                                                          1981

                          S. cerevisiae 197/2d, rad        300-1000 g/ml;                 >98%         Positive            Sharp & Parry, 1981b
                                                           in DMSO                                      (with and without
                                                                                                        activation)

    3.D. Cell Transformation Assays

    Cell transformation   C3H/10T 1/2 cells                100-1000 g/ml;                 99.8%        Negative            McGlynn-Kreft &
                                                           in DMSO                                                          McCarthy, 1984

                                                                                                                                               

    Table 2 (contd)
                                                                                                                                               
    Test system           Test object                      Concentration1                  Purity       Results             Reference
                                                                                                                                               

    Cell transformation   Syrian hamster embryo            62-1000 g/ml                    ?           Negative            Casto, 1975, 1976 
                          (SHE) cells/adenovirus SA7                                                                        in: Heidelberger 
                                                                                                                            et al., 1983

                          Syrian hamster embryo            1-24 mM                          ?           Inconclusive        Hatch et al., 1986
                          (SHE) cells/adenovirus SA7

    Cell transformation   C3H/10T 1/2 cells                33 g/ml; in DMSO               99.8%        Negative            McLeod & Doolittle,
    with "promotion"                                                                                                        1985

    3.E. Germ Cell Effects

    Spermhead             (CBA X BALB/c) F1 mouse          250-2000 mg/kg;                 >98%         Negative            Topham, 1981
    abnormalities                                          in Tween 80

                          B6C3F1/CRL mouse                 166-2655 mg/kg;                 >98%         Negative            Wyrobek et al., 1981
                                                                                                                                                                                                          in DMSO

    1     In vitro assays performed with and without exogenous activation unless indicated otherwise or the test system does not normally
         use such supplementation; solvent is provided if specified in the report
    

    Special studies on the thyroid

    Rats

         Groups of four randomly selected weanling Caesarian-delivered
    Sprague-Dawley male litter-mate rats were administered 0, 75 or 150
    ppm ETU (purity not stated). Within each treatment group dosing
    periods and control diet periods were varied to examine the
    reversibility of compound-related effects.  Results suggest some
    reversibility of thyroid effects which were related to time on test
    and to the severity of effect on the thyroid (Arnold  et al., 
    1982).

         Sprague-Dawley rats (50/sex/group) were fed diets containing 0,
    75, 100 or 150 ppm ETU (purity not stated) mixed in corn oil for 7
    weeks.  Body weights decreased with increasing dose while thyroid
    weights (absolute and relative) increased in both sexes.  T3 levels
    were somewhat variable, while T4 levels were significantly
    decreased at 150 pm in both sexes.  These effects partially reversed
    after 4 weeks on control diets. Histopathological findings included
    reduced colloid content of thyroid acini in high dose rats.  Acinar
    epithelial cell size and height were not different from control. 
    Two tumours were identified in the high dose male group: a
    follicular cell adenoma and medullary carcinoma.  The authors
    concluded that the relationship between the duration of exposure to
    ETU and the possible reversibility of various thyroid lesions
    requires further study (Arnold  et al., 1983).

         A 22-week study was conducted in Sprague-Dawley rats
    (55/sex/group) with the following dosing schedule: ETU (97% purity)
    administered alone in the diet at levels of 0, 125, 250 or 625 ppm;
    or with 0.2 g T3 and 1.6 g T4/rat, orally via gavage; or with
    manganese and zinc.  Also included were treatment groups dosed with
    0, 650 or 1250 ppm mancozeb alone.

         Rats receiving 625 ppm ETU alone or in combination with
    manganese and zinc were removed from test diet because of alopecia,
    weight loss, dermatosis and mortality.  Survivors received control
    diets for the remainder of the study.  Serum decreased in both sexes
    at all doses of ETU after 2 weeks of treatment.  These levels
    returned to normal when ETU was removed from the diet.  Serum T3
    decreased in both sexes at 625 ppm ETU after 4 weeks of dosing, and
    in males at 125 and 250 ppm ETU, but by week 8 returned to normal. 
    In females at the same doses, T3 was normal until week 16 when it
    decreased.  The additions of T3/T4 by oral gavage resulted in
    decreased T3 at week 8 in males and a decrease during the first 6
    weeks in females at all levels.  T3 returned to normal one month
    after removal of ETU.  TSH increased in the ETU group and less
    dramatically in ETU plus T3/T4 groups.  These levels returned to
    normal 2 weeks after ETU was removed form the diet.

         Body weights decreased in males and females after 4 weeks at
    625 ppm ETU and in males after 8 weeks at 250 ppm ETU.  Thyroid to
    body-weight ratio increased at > 125 ppm ETU in males.  When ETU
    was removed from the diet, weights returned to normal.  No effect
    was observed on pituitary weights.  Thyroid hyperplasia was
    increased at 125 ppm ETU and above and reversed to normal 6-8 weeks
    after ETU was removed.  Approximately 1% (13/1300) of the rats
    developed hyperplasia of the thyroid (focal areas of basophilic
    hyperplastic follicles and follicular adenoma).  A dose-related
    increase in liver weight was observed at 125 ppm ETU and above for
    both sexes.

         Exposure to ETU resulted in a decrease in thyroid hormone
    (T3/T4) levels and increased serum TSH levels in a dose-related
    manner.  Although TSH levels were reduced when ETU was supplemented
    with T3/T4, the high dosage of ETU was apparently sufficient to
    override these effects. The hormone imbalance induced by ETU
    correlated with the histologic changes in the thyroid.  Withdrawal
    of ETU from the diet reversed the hypothyroid conditions induced to
    euthyroid (Leber  et al. 1978a).

         In two 90-day feeding trials Sprague-Dawley rats (12/sex/group)
    were given 75 or 100 ppm ETU, and thyroid function, serum T4, T3,
    and TSH, T3 uptake  in vitro, 131I uptake, and thyroid to body-
    weight ratios were measured at days 46 and 91.  Additionally, the
    fate of the incorporated 131I was traced in thyroid fractions at the
    100 ppm level.  Groups of both sexes at the lower feeding level and
    the females at 100 ppm were functionally euthyroid whereas males at
    100 ppm were somewhat hypothyroid despite elevated serum T3, TSH,
    and T3/T4 ratios.  Results showed that the inhibitory effects of
    ETU are similar to those for methimazole.  ETU inhibited
    monoiodotyrosine (MIT) utilization, and the coupling of
    diiodotyrosine (DIT) residues to form T4, resulting in
    significantly reduced active synthesis of T3 and T4 prohormones
    (males 100 ppm).  The capacity of serum to bind T3 was reduced;
    however, there was no evidence for inhibition by ETU of T4 to T3
    monodeiodination or interference with the normal feedback mechanisms
    of thyroid hormones on TSH secretion (O'Neil & Marshall, 1984).

    Observations in humans

         A 53-year old female employed in the manufacture of products
    from synthetic and natural rubber developed itching of the fingers. 
    Her condition improved on weekends but became worse upon returning
    to work.  Two months later the eruption spread to her forearms.  The
    individual showed positive reactions to nickel and cobalt in the
    ICDRG standard patch test as well as to a component of the rubber
    material from her work which was ETU.  Additional patch testing with
    ETU and chemically related substances showed that ETU tested

    positive at dose levels at or above 0.01% (w/v).  One-percent
    concentrations of dibutyl-, diethyl- or diphenylthiourea all were
    negative as were ethyleneurea and ethylenediaminine.  Thiourea at
    0.01% (w/v) was also negative. The fungicides zineb and maneb tested
    negative and positive, respectively.  The positive reaction to maneb
    was attributed to the presence of ETU as an impurity/degradation
    product and confirmed in studies using thin layer chromatography
    (Bruze & Fregert, 1983).

         A retrospective study of women who were employed at a rubber
    manufacturer using ETU was undertaken to determine any excess of
    fetal abnormalities occurring in children to women who had worked
    with ETU during early pregnancy. The women were employed on hand and
    machine trimming, hand cutting, coolant hose manufacture, packing
    and dispatch.  Women were also employed in moulding, cutting blanks
    for moulding and tool and die trimming where there was a hazard from
    inhalation of dust. Women born in 1918 or later who left employment
    between 1963-1971 were surveyed.  Of 699 women who left between
    1963-1971, 255 gave birth to 420 children.  Only 59 of 255 were
    working at the plant at the time of early pregnancy and none of
    these had abnormal children.  Of the total 420 children born 11 had
    abnormalities.  However, figures for the group showed no excess over
    the expected number of fetal abnormalities in the region surveyed.
    The study did not demonstrate any risk of teratogenesis.  However,
    the number of women exposed during early pregnancy was small.

         A total of 1929 workers engaged in the production or
    manufacture of ETU were surveyed retrospectively for thyroid cancer,
    and compared with the thyroid cancer list of the Birmingham
    (England) Cancer Registry from 1957-1971.  No thyroid cancers
    occurred in these workers and the results were considered to be
    preliminary (Smith, 1976).

         An apparent increased incidence of miscarriages among workers
    at a USA rubber products factory was investigated.  Of 7 pregnancies
    only 2 resulted in spontaneous abortions, one with an identified
    medical problem.  Before starting work at the rubber factory, the 81
    women surveyed had 192 pregnancies with 16% spontaneous abortions. 
    There was no indication that menstrual problems increased with
    duration of employment as a moulder. It was concluded that no
    adverse reproductive effects could be attributed to the work
    environment - although the small numbers available for study
    prevented this possibility from being entirely ruled out (Wright  et
     al., 1981).

         No hazard of clinical thyroid depression existed based on
    medical evidence collected on workers exposed to ETU at a rubber
    company in Michigan.  Fifty-one subjects were evaluated (49 males, 2
    females). Environmental sampling results demonstrated that workers
    were exposed to trace amounts of ETU as airborne dust or through
    direct contact of powdered ETU (Salisbury & Lybarger, 1977).

         Clinical examinations and thyroid function tests were carried
    out over a period of 3 years in the United Kingdom on 8 workers
    involved in the manufacture of ETU and 5 workers involved in mixing
    of ETU with rubber. The average length of exposure in workers
    associated in the manufacturing process was 10 years with a range of
    5-20 years. The exposure period for mixers was 3 years.  All
    subjects were males with an age range from 26-62 years.  Matched
    controls were also examined.  In the manufacturing plant ETU levels
    of 330 g/m3 were recorded on one personal sampler. Background
    levels ranged from 10-240 g/m3.  Levels of ETU recorded on
    personal samplers of mixers ranged from 120-160 g/m3.  Mixers but
    not process workers had significantly lower levels of T4 in their
    blood compared to controls.  No effects were found on TSH or thyroid
    binding globulin.  The authors concluded that there was no evidence
    that thyroid function is severely affected by exposure to ETU at the
    levels experienced by these workers nor was there any evidence of
    any effect.  However, the T4 results in the exposed workers were
    generally lower than those in the control group with most of the
    difference in distribution accounted for by the results from the
    mixers (Smith, 1984).

         The concentration of ETU in pesticide formulations and ambient
    air were measured and exposure to maneb (80% powder) or mancozeb
    (Ridomil 56% mancozeb) evaluated during the spraying of potato
    fields. The mean tank mix concentrations of maneb and mancozeb were
    4.0 or 7.0 g/litre, respectively.  Spraying time was 0.5-7.0 hours
    (mean 4.0 hours).  Each single application was separated by several
    weeks.  Therefore only acute (single day) exposure was determined in
    workers (i.e. mixer/loader/applicator).

         The overall range of concentrations of ETU in air were between
    0.004 and 3.3 g/m3 in the breathing zone and 0.006 and 0.8 g/m3
    in the (closed) tractor cabin.  The mean calculated concentrations
    of active maneb and mancozeb in air were 7 and 20 g/m3,
    respectively. The total inhaled amount of ETU and EBDCs was 5.0 and
    126.0 g/day, respectively corresponding to a dose of 0.07 g ETU/kg
    bw and 1.8 g EBDC/kg bw for applicators and mixer and loaders
    weighing 70 kg.  Patch samples on clothes and skin (back, chest,
    shoulders and forearm) indicated that 1.4, 10.0, 4.0 and 1.2% of ETU
    (0.07; 0.19; 0.39; 0.17 mg/cm2/hour) reached the skin respectively. 
    ETU in urine sample taken on days 1, 8, 15 and 22 ranged between
    0.09-2.5; 0.07-1.0; 0.01-0.30 and < 0.01-0.2 g/mmol creatinine. 
    Absolute concentrations of ETU in all samples ranged between < 0.2
    and 11.8 g/litre of urine.  Under the exposure conditions the
    urinary elimination half-life was calculated to be 100 hours
    (Kurrtio  et al., 1990).

    COMMENTS

         Following oral administration to mice essentially all of the
    ETU was recovered in the excreta within 48 hours; none was recovered
    as carbon dioxide.  Approximately 50% of the administered dose was
    found in urine as unchanged ETU.

         After the oral administration of radiolabelled ETU, its
    concentration in both pregnant mice and rats peaked about the same
    time (1.4 hours), with concentrations in maternal and fetal tissues
    similar at 3 hours.  The half-life of elimination from mice and rats
    was 5.5 hours and 9.4 hours, respectively.  Approximately 70% of ETU
    was found in urine in both species at 48 hours.

         Mice metabolize ETU primarily by the flavin-monooxygenase
    system and rats by the P-450 system of enzymes.

         ETU is slightly toxic after acute oral administration, with the
    LD50 ranging from 545 mg/kg bw in pregnant rats to 4000 mg/kg bw in
    adult mice.

         In a 13-week study in mice at dietary concentrations of 0, 125,
    250, 500, 1000 or 2000 ppm the NOAEL was 250 ppm (equivalent to 38
    mg/kg bw/day). Diffuse follicular cell hyperplasia of the thyroid
    and hepatocellular cytomegaly were observed at 500 ppm.

         In a three-month study in mice at dietary concentrations of 0,
    1, 10, 100 or 1000 ppm the NOAEL was 10 ppm, equal to 1.7 mg/kg
    bw/day. ETU produced thyroid follicular cell hyperplasia and
    decreased colloid density at 100 ppm.

         The NOAEL in a study in which rats were fed dietary
    concentrations of ETU at 0, 0.63, 1.3, 2.5, 5.0 or 25 ppm for 8
    weeks was 25 ppm (equal to 2.6 mg/kg bw/day), the highest dose
    tested.

         In a 13-week study in rats, ETU was administered in the diet at
    concentrations of 0, 60, 125, 250, 500 or 750 ppm.  The NOAEL was
    less than 60 ppm (equal to 3.0 mg/kg bw/day) based on
    histopathological findings of diffuse follicular cell hyperplasia in
    the thyroid.

         In a 90-day study in rats, ETU was administered in the diet at
    concentrations of 0, 1, 5, 25, 125 or 625 ppm.  The NOAEL was 25 ppm
    (equal to 1.7 mg/kg bw/day) based on hyperaemia of the thyroids,
    with and without enlargement, increased thyroid to brain weight
    ratio, decreased 125I thyroid uptake, decreased triiodothyronine,
    decreased thyroxine and increased thyroid follicular cell
    hyperplasia at 125 ppm.

         In a four-week feeding study in dogs at dietary concentrations
    of 0, 200, 980 or 4900 ppm, the NOAEL was 200 ppm, equal to 6.7
    mg/kg bw/day.  Decreased body-weight gain, decreased thyroxine and
    T3 levels and enlarged thyroids were observed at 980 ppm.

         In a 13-week feeding study in dogs at dietary concentrations of
    0, 10, 150 or 2000 ppm the NOAEL was 10 ppm, equal to 0.39 mg/kg
    bw/day.  At 150 ppm haemoglobin, packed cell volume, and red blood
    cell count were decreased, and cholesterol was increased.  Effects
    on the thyroid were found only at 2000 ppm.

         In a 52-week feeding study in dogs at dietary concentrations of
    0, 5, 50, or 500 ppm, the NOAEL was 5 ppm, equal to 0.18 mg/kg
    bw/day. At 50 ppm a reduction in body-weight gain, hypertrophy of
    the thyroid with colloid retention, a slight increase in thyroid
    weight and pigment accumulation in the liver were observed.

         Male and female mice received perinatal (F0) and adult (F1)
    exposure to ETU at the following dietary concentrations (F0,F1);
    0,0; 0,330; 0,1000; 330,0; 330,330; 330,1000; 110,330 or 33,100 ppm. 
    Mice receiving perinatal exposure only (330,0 ppm) showed no effect
    on the incidences of neoplasms after 2 years.  Cytoplasmic
    vacuolization of follicular cells of the thyroid was evident in
    males and females at 33,100 ppm, but no increases in neoplasms of
    the liver, pituitary or thyroid were observed.  T4 values were
    significantly decreased in both sexes and thyrotropine was slightly
    elevated.  Animals receiving 330 ppm during adulthood showed tumours
    of either the liver, pituitary or thyroid.  Increasing perinatal
    exposure from 0 to 330 ppm was associated with an increased
    incidence of thyroid and pituitary lesions in female mice receiving
    adult exposure to 330 ppm, but there were no enhancing effects of
    perinatal exposure in mice receiving adult exposures of 1000 ppm
    when compared to adults in the 0,1000 ppm group.

         Rats were fed dietary concentrations of ETU at levels of 0, 5,
    25, 125, 250 or 500 ppm for 2 years.  The NOAEL was 5 ppm,
    equivalent to 0.25 mg/kg bw/day.  Vascularity and hyperplasia of the
    thyroid were seen at 25 ppm.

         In a two-year feeding study in rats using dietary
    concentrations of 0, 0.5, 2.5, 5 or 125 ppm the NOAEL was 5 ppm
    (equal to 0.37 mg/kg bw/day) based on changes in clinical chemistry,
    increased triiodothyronine, decreased thyroxine, increased thyroid
    weight, increased liver weight and an increased incidence and
    severity of diffuse thyroid follicular cell hyperplasia at 125 ppm.

         In a two-year carcinogenicity study in rats using dietary
    concentrations of 0, 175 or 350 ppm, thyroid carcinomas and
    hyperplastic goitres were observed in both sexes at 175 ppm
    (equivalent to 8.8 mg/kg bw/day).

         Male and female rats received perinatal (F0) and adult (F1)
    exposure to ETU at the following dietary concentrations (F0,F1);
    0,0; 0,83; 0,250; 90,0; 90,83; 90,250; 30,83 or 9,25 ppm. Rats
    receiving perinatal and adult exposure of 9,25 ppm showed no
    increase in tumours and no apparent biologically meaningful changes
    in thyroid hormone function at two-years when compared to 0,0 ppm
    controls.  Thyroid hyperplasia was evident in both sexes.  At 9
    months, animals given 9,25 ppm manifested decreased T3 and T4
    values and increased thyrotropine without evidence of thyroid
    follicular cell hyperplasia.  Males and females receiving a dose of
    90,0 ppm showed no hormonal changes and no tumours at 2 years. 
    Thyroid follicular cell hyperplasia was, however, evident.  Animals
    receiving adult exposure showed a significant increase in thyroid
    follicular cell tumours at 83 and 250 ppm (males) and 250 ppm
    (females).  Males and females showed no significant differences in
    the number of tumours between dose groups of 0,83; 30,83; and 90,83
    ppm.  Males and females receiving 90,250 ppm showed increases in
    thyroid follicular cell tumours when compared to 0,250 ppm.  At the
    end of 2 years males and females receiving 0,83 or 0,250 manifested
    increased numbers of thyroid tumours when compared to 0,0 ppm
    controls.

         In a two-generation reproduction study in rats at dietary
    concentrations of 0, 2.5, 25 or 125 ppm the NOAEL was 2.5 ppm, equal
    to a range of 0.16-0.38 mg/kg bw/day, based on thyroid gland
    follicular cell hyperplasia and hypertrophy at 25 ppm.

         An oral teratogenicity study conducted in rats at dose levels
    of 0, 5, 10, 20, 40 or 80 mg/kg bw/day indicated no maternal
    toxicity at 40 mg/kg bw/day (NOAEL).  Maternal lethality was
    observed at 80 mg/kg bw/day.  The NOAEL for embryo/fetotoxicity
    effects was 5 mg/kg bw/day based on teratogenic effects observed at
    10 mg/kg bw/day.

         An oral teratogenicity study in rats at dose levels of 0, 15,
    25 or 35 mg/kg bw/day was conducted.  No maternal toxicity was
    observed at 35 mg/kg bw/day (NOAEL).  The NOAEL for
    embryo/fetotoxicity and teratogenicity was 15 mg/kg bw/day based on
    higher incidences of dilated brain ventricles at 25 mg/kg bw/day.

         Oral teratogenicity studies in rats (0, 10, 20, 30, 40 or 50
    mg/kg bw/day), mice (0, 200, 400 or 800 mg/kg bw/day) and hamsters
    (0, 90, 270 or 810 mg/kg bw/day) revealed no maternal toxicity at
    the doses tested.  The NOAEL for embryo/fetotoxicity in the rat was
    10 mg/kg bw/day based on dilation of the lateral or fourth ventricle
    at 20 mg/kg bw/day.  The NOAEL for embryo/ fetotoxicity in the
    hamster was 90 mg/kg bw/day based on a decrease in fetal body weight
    at 270 mg/kg bw/day.  The NOAEL for mice was greater than 800 mg/kg
    bw/day.

         In an oral teratogenicity study, rabbits received 0, 5, 10, 20,
    40 or 80 mg/kg bw/day of ETU.  The NOAEL for maternal toxicity was
    80 mg/kg bw/day. The NOAEL for embryo/fetotoxicity was 40 mg/kg
    bw/day based on an increase in resorption sites, decreased brain
    weight and a degeneration of the proximal convoluted tubules in the
    kidneys of fetuses at 80 mg/kg bw/day.  Malformations were not
    observed at the highest dose.

         A study with pregnant rats administered ETU, T3/T4 and sodium
    iodide in combination indicated a reduction in some of the
    teratogenic responses when compared with groups administered ETU
    alone.  These results indicate that the teratogenic potential of ETU
    may in part be secondary to the thyroid toxicity of ETU.

         ETU has been the subject of many  in vitro and   in vivo
    studies for genotoxicity.  It induces mutations in bacteria at very
    high doses but variable responses have been obtained in other types
    of mutation assays.  Acceptable assays for other genotoxicity
    endpoints  in vitro were generally negative, while all  in vivo
    assays were negative.  The Meeting concluded that ETU was not
    genotoxic.

         An ADI was allocated based upon a NOAEL of 0.39 mg/kg bw/day in
    the 13-week study in dogs, since this dose level was between the
    NOAEL of 5 ppm (equal to 0.18 mg/kg bw/day) and the middle dose
    (effect level) of 50 ppm (equal to 1.8 mg/kg bw/day) in the 52-week
    dog study.  A 100-fold safety factor was applied.

    TOXICOLOGICAL EVALUATION

    Level causing no toxicological effects

         Mouse:    10 ppm, equal to 1.7 mg/kg bw/day (3-month study)

         Rat:      5 ppm, equal to 0.37 mg/kg bw/day (two-year study)
                   2.5 ppm, equal to a range of 0.16-0.38 mg/kg bw/day
                   (reproduction study)

         Dog:      10 ppm, equal to 0.39 mg/kg bw/day (13-week study)
                   5 ppm, equal to 0.18 mg/kg bw/day (52-week study)

    Estimate of acceptable daily intake for humans

         0-0.004 mg/kg bw.

    Studies which will provide valuable information in the continued
    evaluation of the compound

         Observations in humans.

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    Legator, M.S., Bueding, E., Batzinger, R., Connor, T.H.,
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    Lewerenz, H.J. & Plass, R. (1984). Contrasting effects of
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    Lu, M.H. & Staples, R.E. (1978). Teratogenicity of ethylenethiourea
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    Marshall, W. (1977). Thermal decomposition of
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    Martin, C.N. & McDermid, A.C. (1981). Testing of 42 coded compounds
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    McGregor, D., Brown, A., Cattanach, P., Edwards, I., McBride, D,
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    Mehta, R.D. & von Borstel, R.C. (1981). Mutagenic activity of 42
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    Mohn, G.R., Vogels-Bouter, S. & van der Horst-van der Zon, J.
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    Mortelmans, K., Haworth, S., Lawlor, T., Speck, W., Tainer, B. &
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    Natarajan, A.T. & van Kesteren-van Leeuwen, A.C. (1981). Mutagenic
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    O'Neil, W. & Marshal, W. (1984). Goitrogenic effects of
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    Paika, I.J., Beauchesne, M.T., Randall, M., Schreck, R.R. & Latt,
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    Parry, J. (1986). The expression of recessive markers in presumptive
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    Parry. J.M. & Sharp, D.C. (1981). Induction of mitotic aneuploidy in
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    Perry, P.E. & Thomson, E.J. (1981). Evaluation of the sister
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    Peters, A.C., Kurtz, P.J., Donorrio, D.J., Thake, D.C. & Cottrill,
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    Peters, A.C., Kurtz, P.J., Donorrio, D.J., Thake, D.C. & Cottrill,
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    Peters, A.C., Kurtz, P.J., Chin, A.E., Carlton, B.D., Chrisp, C.E.,
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    Pilinskaya, M. (1982) Analysis of the cytogenetic activity of
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    Resnick, M.A., Mayer, V.W., and Zimmermann, F.K. (1986). The
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    Richold, M. & Jones, E. (1981). Mutagenic activity of 42 coded
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    Robinson, D.E. & Mitchell, A.D. (1981). Unscheduled DNA synthesis
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    Rosencranz, H.S., Hyman, J. & Leifer, Z. (1981). DNA polymerase
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    Rowland, I. & Severn, B. (1981). Mutagenicity of carcinogens and
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    Ruddick, J.A. & Khera, K.S. (1975). Pattern of anomalies following a
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    Ruddick, J.A., Williams, D.T., Hierlihy, L. & Khera, K.S. (1976).
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    Ruddick, J.A., Newsome, W.H. & Iverson, F. (1977). A comparison of
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    Saillenfait, A.M., Sabate, J.P., Longonne, I. & De Ceaurriz, J.
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    Salamone, M.F. (1981). Mutagenic activity of 41 compounds in the  in
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    Savolainen, K. & Pyysalo, H. (1979). Identification of the main
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    Schiestl, R., Gietz, R.D., Mehta, R. & Hastings, P. (1989).
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    Schupbach, M. & Hummler, H. (1977). A comparative study of the
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    Seiler, J.P. (1975).  In vivo mutagenic interaction of nitrite and
    ethylenethiourea.  Experientia, 31: 214-215.

    Seiler, J.P. (1977a). Inhibition of testicular DNA synthesis by
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    Seiler, J. (1977b). Nitrosation  in vitro and  in vivo by sodium
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    Sharp, D.C. & Parry, J.M. (1981a). Induction of mitotic gene
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    Sharp, D.C. and Parry, J.M. (1981b). Use of repair-deficient strains
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    Shirasu, Y., Moriya, M., Kato, K., Lienard, F., Tezuka, H.,
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    Simmon, V.F. & Shepherd, G. F. (1981). Mutagenic activity of 42
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    Smith, D.M. (1976). Ethylenethiourea - a study of possible
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    Sram, R.J. (1975). Genetic risk from chemicals: mutagenicity studies
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    Stula, E.F. & Krauss, W.C. (1977). Embryotoxicity in rats and
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    Styles, J.A. (1981).  Activity of 42 coded compounds in the BHK-21
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    Teramoto, S., Shingu, A. & Shirasu, Y. (1978b). Induction of
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    Thomson, J.A. (1981). Mutagenic activity of 42 coded compounds in
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    Tophan, J.C. (1981). Evaluation of some chemicals by the sperm
    morphology assay.  Prog. Mutation Research, 1: 718-720.

    Trueman, R..W. (1981). Activity of 42 coded compounds in the
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    Tsuchimoto, T. & Matter, B.E. (1981). Activity of coded compounds in
    the micronucleus test.  Prog. Mutation Research, 1: 705-711.

    Tweats, D.J. (1981). Activity of 42 coded compounds in a
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    Valencia, R. & Houtchens, K. (1981). Mutagenic activity of 10 coded
    compounds in the  Drosophila sex-linked recessive lethal test.
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    NIOSH Health Hazard Evaluation Report HETA 81-270-1012. December,
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    Wyrobek, A., Gordon, L. & Watchmaker, G. (1981). Effect of 17
    chemical agents including 6 carcinogen/noncarcinogen pairs of sperm
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    See Also:
       Toxicological Abbreviations