Chlorpyrifos (O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl)-phosphorothioate) was evaluated for potential developmental toxicity. Groups of 30 bred female Fischer 344 rats were given 0, 5, 15, and 25mg/kg per day by gavage on gestation days 6-15; the fetuses were evaluated on gestation day 21. Clinical signs of toxicity attributed to chlorpyrifos were noted in dams receiving 15 and 25mg/kg per day. Maternal effects in these groups also included depressed body weight and acetylcholinesterase activity. Fetal weight and viability were decreased, and fetal death and early resorption were increased at the 25mg/kg per day maternal dose. Visceral, skeletal, and external variations were also increased in this group. Chlorpyrifos showed fetotoxic and teratogenic effects at a maternal dose of 25mg/kg per day, a dose that also produced maternal toxicity.