The effects of gestational exposure to the commonly used organophosphorus insecticide chlorpyrifos (O,O-diethyl O-[3,5,6-trichloro-2-pyridinyl]phosphorothioate) on postnatal central and peripheral cholinergic neurochemistry were investigated. Pregnant rats were orally dosed daily with chlorpyrifos (0, 3, 5, or 7 mg/kg) in corn oil from gestation day 6 to 20. Pups were sacrificed on postnatal days 1, 3, 6, 9, and 12 for the determination of brain, heart, lung, and serum cholinesterase, and brain choline acetyltransferase activities, along with liver carboxylesterase activity. Exposure to chlorpyrifos did not produce signs of overt toxicity to the dams or developing offspring. Cholinesterase activities were inhibited in a dose-related manner, with brain cholin-esterase inhibition of about 26%, 32%, and 45% on postnatal day 1. Inhibition of brain cholineste-rase persisted in all treatment groups until postnatal day 6 and in the medium and high-dosage groups through postnatal day 9. Liver carboxylesterase activity was also inhibited in a dose-related manner, with a recovery profile parallel to that of brain cholinesterase. Choline acetyltransferase activity was decreased by about 13% in the high-dosage group on postnatal days 9 and 12. These results indicate that gestational exposure to chlorpyrifos results in relatively persistent inhibition of brain cholinesterase and a delayed depression of choline acetyltransferase at a time when brain cholinesterase activity had returned to control levels in the high-dosage group.