When chemical agents penetrate the placenta, it is potentially hazardous to the embryo because the embryonic stage is known to be extremely sensitive to various toxic agents. It has been reported that exposure to some chemical agents during pregnancy resulted in the induction of malformation or cancer in the offspring of experimental animals (Larsen, 1947; Klein, 1952; DiPaolo, 1964; Druckrey et al. 1966; Mohr et al, 1966; DiPaolo and Elis, 1967; Spatz and Laqueus, 1967; Alexandrov, 1968; Fujii and Nishimura, 1969; Rice, 1969; Bulay and Wattenberg, 1970; Currie, 1970; Vesselinovitch et al, 1971; Swenberg et al, 1972; Nomura et al, 1973). Fraser and Fainstat (1954) and Kalter (1954) found that administration of cortisone to pregnant female mice induced the appearance of cleft palates in the offspring. The frequency with which this deformity appears was observed to depend on: I) the genotype of the treated animal (strain differences), 2) the dose of the chemical administered, 3) the time during the gestation period when the animal was treated. A single intraperitoneal injection of 5-fluorouracil at 10, 11, 12 or 13 days after copulation in mice also produced abnormalities to the feet, deft palate and deformities of the tail in a large proportion of fetuses (Dagg, 1960). Urethan has been considered to be a highly teratogenic and carcinogenic agent in experimental animals (Nishimura and Kuginuki, 1958: Nomura and Okamoto, 1972). However, they stated that accurate timing of urethan toxicity and accurate calculation of urethan dosage actually reaching the embryo make it possible to analyze the sensitivity of the developing mouse embryo to mortality, growth inhibition, malformation and neoplasm. Nomura and Okamoto (1972) reported that when pregnant mice were exposed to urethan on various days of gestation (day 5 to 19) by a single injection malformations and neoplasms were induced in their offspring. It is frequently implied that an abnormal phenotype is due to the aberration in the genotype, but it is not possible to prove the specitic causal relation. Though, the frequent association between a variety of chromosomal abnormalities solves the problem of how the genotypic and phenotypic are interreiated (Schultz, 1965). 5-bromodeoxyuridine (BUdR) and dimethylnitrosamine (DMN) induce chromosome aberrations in Chinese hamster cells cultured human lymphocytes and mouse cells in vivo (Somers and Hus, 1962; Kato, 1968; Matsuoka et al, 1979; Hahn and Kim, 1979). BUdR is a thymidine analog incorporated into only the DNA of proliferating cells and its mutagenic action is well understood (Freese, 1963). DMN is a potent carcinogen which induces tumors of the liver, lung, and kidney in rats (Magee and Bames, 1959). This agent has no teratogenic effect in rats when given in doses of different concentrations for different periods of time and by several routes of administeration during all stages of embryogeny (Alexandrov, 1967). The experiments reported in this study were undertaken to investigate the possibility that treatment of ICR inbred pregnant mice with BUdR and DMN might shows deformities or abnormalities in their offspring and also to determine whether chemical exposure during fetus will effect at 32 weeks after birth with second exposure to DMN by cytogenetical means. In this study, estrus ICR females were mated and 32 mice which had been diagnosed as pregnant were used. BUdR at the rate of 70, 100 and 150mg/kg of body weight was injected intraperitoneally at 6, 7, 8 days and 9, 11, 13 days of gestation, and DMN at the rate of 10, 20 and 30 mg/kg of body weight was injected at 8, 10, 12 days and 14, 15, 16 days of gestation, The offspring were examined macroscopicaily at time of birth for malformations. All animals were killed at 32 weeks of age and examined for liver abnormalities. The liver were cultured and treated with 1, 5 and 10 ug/ml of DMN for 18 hours. The frequencies of chromosome aberrations and sister chromatid exchanges (SCE) were analyzed. The results are summarized as follows: 1. The litter size was reduced on treated animals. 2. Among the 279 progeny from 36 BUdR treated mothers, malformations were seen in a total of 10 progeny and the group treated at the 9 to 13 gestation days stage had the most. 3. Of the 155 progeny from 24 mothers injected with DMN, none had any visible deformity. However. 37.5% of the group were found to have liver nodules after 32 weeks treated at the 8 to 12 gestation day stage. 4. Repetitive treatment with DMN of the liver culture of the previously BUdR and DMN treated progeny, showed increased chromosome aberrations and SCE frequencies. In conclusion since the exposure of the mother of BUdR and DMN during pregnancy leads to increased chromosomal abnormalities of the cultured liver cells of progeny when treated with DMN a second time, it is necessary to keep in mind that genetic damage may be occure to the progeny by exposing the mother during pregnancy.