Exposure of developing animals to apparently subtoxic doses of chlorpyrifos (CPF) during a critical period of synaptogenesis has been shown to affect catecholaminergic synaptic development and neuronal activity separably from its inhibition of cholinesterase. We used rat brain synaptosome preparations to examine whether CPF has a direct effect on the release of norepinephrine (NE). Synaptosomes were preloaded with [3H]NE in the absence of CPF and were then exposed to the compound during subsequent neurotransmitter release. There was a robust increase in release at 50 microg/ml of CPF. The effect was not mediated through cholinergic receptors, as neither atropine nor mecamylamine interfered with the actions of CPF. Enhanced NE release was seen in synaptosomes derived from neonatal rat brain as well as adult rat brain, albeit with a smaller effect in neonates. Our results suggest that CPF interacts directly with presynaptic nerve terminals to influence neurotransmitter release; in the context of the immature brain, these effects can alter synaptic development through the trophic actions of catecholamines.