Acephate (O,S - dimethyl acetyl phosphoramidothioate), an organophosphate insecticide, was evaluated for its potential to produce developmental toxicity in mice after oral administration. Pregnant ICR (CD-1) mice were given sublethal doses of 0 (distilled water), 7, 14, and 28 mg/kg/day acephate by gavage on Gestation Days 6 through 15. Maternal effects in the 28 mg/kg/day dose group included cholinergic signs, decreased body weight at 15 and 18 days of gestation, and decreased absolute and relative brain weight. Placental weight was also decreased and liver weight was increased in the high dose group. Absolute and relative brain weight was decreased in the 14 mg/kg/day group. No maternal effects were apparent in the 7 mg/kg/day dose group. Maternal exposure to acephate during organogenesis significantly affected the number of implantations, number of live fetuses, number of early resorptions, mean fetal weight, and the incidence of external and skeletal malformations in the 28 mg/kg/day dose group. No visceral malformations were observed. On the basis of the present results acephate showed maternal and developmental toxicity at 28 mg/kg/day.