The biochemical and morphological neurotoxic effects of postnatal acetylcholinesterase (AChE) inhibition were examined in rat pups dosed with parathion, at time points critical to hippocampal neurogenesis and synaptogenesis (i.e., D5-20). Hippocampal cytopathology as assessed by light and electron microscopy, consisted of cellular disruption and necrosis in the DG, CA4, and CA3a regions. Synaptic disruption in the DG molecular layer was suggested by histochemical preparation using both the Timm's and AChE stains. In parathion-treated D12 pups, hippocampal AChE was depressed 70% and QNB binding depressed by 36%. The above results indicated that definite neurotoxic consequences are associated with persistent AChE depression in the neonatal rat.