Pregnant ICR-JCL mice were given 6-aminonicotinamide (6-AN) in a dose of 8 and 15mg/kg orally on the days 7 and 9,respectively. The animals were sacrificed on day 18 of gestation and the fetuses examined for external, visceral and skeletal anomalies in cleared specimens stained with alizarin red S. Weight of all the pregnant mice decreased after treatment with the 6-AN. The fetal mortality rate after administration of 6-AN was significantly high in all groups, as compared with the controls. When two pregnant mice were given 15mg/kg on day 7,resorptions in the uterus were seen on day 18. The mean body weight of living mice from the administered dams was dependent on the day of administration. There were relationships between the incidence of congenital anomalies and the the day of administration, and the dose level. In the skeleton of the fetuses from dams ingesting 6-AN, many anomalies were concentrated in the lumbosacral region and lower extremities. Congenital hydronephrosis was seen in two fetuses, when 8mg/kg was given to the dams on day 7.