Aspalatone, a new antithrombotic agent, was administered orally to pregnant Sprague-Dawley rats during the organogenetic period at dose levels of 0, 20, 100 and 500 mg/kg/day. All dams were subjected to caesarean section on day 20 of pregnancy. Effects of test substance on dams and embryonic development of F1 fetuses were examined There were treatment-related decreases in body weight and food consumption in the 500 mg/kg group. There was a increase in the spleen weight in the 100 and 500 mg/kg groups. Develo-pmental toxicity was evident as decreased fetal body weights and increased fetal malformations in the 500 mg/ kg group. External and skeletal malformations of fetuses occurred at an incidence of 1 and 8.2%, respectively. In addition, there was a delay in ossification of sternebrae and sacrocaudal vertebrae in the 500 mg/kg group. The results show that the no observed adverse effect dose level (NOAEL) for maternal toxicity was 20 mg/kg/ day and for developmental toxicity was 100 mg/kg/day.