We re-examined the teratogenicity of enflurane and sevoflurane using A/J strain mice on the 11th day of gestation. Exposure conditions and observation period: Based on results of preliminary examinations, animals in the 11th day of gestation were exposed to enflurane and sevofluran for 8 hours under three different conditions of air (approximately 21% O2 concentration), air and oxygen (approximately 60% O2 concentration) and pure oxygen (100% concentration at 2 liters/min). Cesarean section was performed on the 18th day of gestation and the fetus was examined for the presence of abnormalities. The results indicated that under these conditions, enflurane and sevoflurane caused the occurrence of cleft palate in the same manner as halothane. However, the rate of occurrence was lower than that of halothane and there were no cases of fetuses with cleft palate observed in ddY strain mice which served as the control group. Further, it was suggested that in the case of cleft palate seen in high liability groups such as A/J strain mice, it is possible to prevent to a certain extent the occurrence of cleft palate by increasing the concentration of oxygen in the anesthetic.