AbstractTeratogenic effects of the anticonvulsant drugs valproic acid (VPA) and diphenylhydantion (DPH) on the development of mouse embryos during early organogenesis were studied using the whole embryo culture technique. Embryos with one to seven somites were exposed in vitro to 50–375 μg/ml VPA or 15–135 μg/ml DPH for up to 42 hours and compared to control embryos cultured in 80% rat serum without either drug. For both VPA‐ and DPH‐treated embryos, a dose‐dependent increase in the frequency of abnormal embryos and a decrease in viability were found. VPA and DPH produced a similar pattern of defects. Drug‐induced anomalies included open neural tubes in the cranial regions, abnormal body curvature, craniofacial deformities, and yolk sac defects. Ultrastructural changes were noted in the neuroepithelium of exencephalic VPA‐treated embryos. Growth and development were retarded in embryos exposed to>35 μg/ml DPH or>50 μg/ml VPA as indicated by the decrease in protein and DNA content and the reduction in somite number, crown‐rump length, and yolk sac diameter. On a molar basis DPH was potentially more teratogenic than VPA, which correlates with the higher lipid solubility of DPH. With VPA, susceptibility to the drug depended on the developmental stage; e.g., at 150 μg/ml VPA the frequency of malformations was 70% in embryos with one to four somites as compared to 35% in embryos with f