Hydroxyurea (HU), a ribonucleotide reductase inhibitor, induces morphological anomalies in the central nervous system, craniofacial tissues and limb buds in animals, and neonatal respiratory distress in humans. The neonates and offspring of pregnant mice treated with 400 or 800 mg/kg of HU on day 13 of gestation were examined at 0 day and 10 weeks after birth to find a clue for clarifying the relationship between HU-induced apoptosis in the fetal tissues and teratogenicity. The offspring from dams treated with HU were retarded in growth compared with controls. But there was no significant difference in the body weight gain between the 400 and 800mg/kg groups. In the teratologic changes, microencephaly, hydrocephalus and curved coccygeal vertebrae were observed in the offspring, and the incidence of these teratologic changes was similar but their degree was more severe in the 800 mg/kg group than in the 400 mg/kg group. Based on the above-mentioned previous and present studies of ours, we suggest that HU-induced apoptosis in fetal tissues may play an important role in the development of anomalies in the corresponding tissues of offspring.