M73101 and aspirin were evaluated for the effects on prenatal development of progeny in JW-NIBS strain rabbits. Pregnant females were given orally M 73101 (0, 50, 120 and 300mg/kg/day) or aspirin (200mg/kg/day) on days 6 to 18 of gestation. They were sacrificed on days 29 of pregnancy and their fetuses were examined for abnormalities. The results were summarized as follows : 1. During the dosing period, females received 300mg/kg of M73101 showed pronounced body weight depression and decrease in food and water intake. But after the dosing period, these influences were not seen. Females received aspirin did not show such phenomena. 2. There were no adverse effects on number of implants, litter size, fetal mortality or fetal weight, and no malformed fetuses attributable to the drugs were seen in each group.