Pregnant Fischer 344 rats and New Zealand White rabbits were orally administered 0, 5, 15, or 50 mg nitrapyrin/kg/day on Gestation Days 6 through 15 (rats) or 0, 3, 10, or 30 mg/kg/day on Gestation Days 6 through 18 (rabbits). In rats, 50 mg/kg/day produced slight histopathologic changes in the livers of pregnant females. Fetal examination revealed no evidence of fetotoxicity or teratogenicity among rats at dose levels up to 50 mg/kg/day. Among rabbits, a significant depression in maternal weight gain and increased absolute and relative liver weights were observed at 30 mg/kg/day. An increased incidence of crooked hyoid bone among fetal rabbits in the 30 mg/kg/day dose group was considered indicative of fetotoxicity but not teratogenicity. Thus, administration of nitrapyrin was not teratogenic at dose levels up to 50 mg/kg/day in rats and 30 mg/kg/day in rabbits.