Neonatal mice were treated daily on postnatal days (pnds) 1 through 4 or 11 through 14 with the organophosphate pesticide chlorpyrifos (CPF), at doses (1 or 3 mg/kg) that do not evoke systemic toxicity. Brain acetylcholinesterase (AChE) activity was evaluated within 24 h from termination of treatments. Pups treated on pnds 1-4 underwent ultrasonic vocalization tests (pnds 5, 8, and 11) and a homing test (orientation to home nest material, pnd 10). Pups in both treatment schedules were then assessed for locomotor activity (pnd 25), novelty-seeking response (pnd 35), social interactions with an unfamiliar conspecific (pnd 45), and passive avoidance learning (pnd 60). AChE activity was reduced by 25% after CPF 1-4 but not after CPF 11-14 treatment. CPF selectively affected only the G(4) (tetramer) molecular isoform of AChE. Behavioral analysis showed that early CPF treatment failed to affect neonatal behaviors. Locomotor activity on pnd 25 was increased in 11-14 CPF-treated mice at both doses, and CPF-treated animals in both treatment schedules were more active when exposed to environmental novelty in the novelty-seeking test. All CPF-treated mice displayed more agonistic responses, and such effect was more marked in male mice exposed to the low CPF dose on pnds 11-14. Passive avoidance learning was not affected by CPF. These data indicate that developmental exposure to CPF induces long-term behavioral alterations in the mouse species and support the involvement of neural systems in addition to the cholinergic system in the delayed behavioral toxicity of CPF.