Chlorpyrifos (CPS; O,O-diethyl 3,5,6-trichloro-2-pyridyl phosphorothionate; Dursban) is a widely used broad-spectrum organophosphorus (OP) insecticide. Because some OP compounds can cause a sensory-motor distal axonopathy called OP compound-induced delayed neurotoxicity (OPIDN), CPS has been evaluated for this paralytic effect. Early studies of the neurotoxicity of CPS in young and adult hens reported reversible leg weakness but failed to detect OPIDN. More recently, a human case of mild OPIDN was reported to result from ingestion of a massive dose (about 300 mg/kg) in a suicide attempt. Subsequent experiments in adult hens (the currently accepted animal model of choice for studies of OPIDN) showed that doses of CPS in excess of the LD50 in atropine-treated animals inhibited brain neurotoxic esterase (NTE) and produced mild to moderate ataxia. Considering the extensive use of CPS and its demonstrated potential for causing OPIDN at supralethal doses, additional data are needed to enable quantitative estimates to be made of the neuropathic risk of this compound. Previous work has shown that the ability of OP insecticides to cause acute cholinergic toxicity versus OPIDN can be predicted from their relative tendency to inhibit the intended target, acetylcholinesterase (AChE), versus the putative neuropathic target, NTE, in brain tissue. The present study was designed to clarify the magnitude of neuropathic risk associated with CPS exposures by measuring hen brain AChE and NTE inhibition following dosing in vivo and determining the bimolecular rate constant of inhibition (ki) for each enzyme by the active metabolite, CPS oxon (CPO), in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)