The toxicity of trichlorfon (O, O-dimethyl-2,2,2,-trichloro-1-hydroxyethylphosphonate, Dipterex®, Dylox®), reported to elicit delayed neurotoxicity in man and chickens, was studied by administering single subcutaneous doses of 100 or 300 mg/kg to adult White Leghorn hens. At 24 h posttreatment, the birds were observed for visible signs of neurotoxicity, were euthanized, and samples of blood plasma, brain, and spinal cord (cervical and thoracic regions) were obtained for quantification of cholinesterase and neurotoxic esterase (NTE) activities. In subacute studies, hens were dosed with trichlorfon (100 mg/kg) every 72 h for a total of six doses. Seventy-two hours after the final dose the hens were euthanized, the brains, spinal cords, and distal sciatic nerves were removed for enzymatic and (or) histological examination. Parallel acute and subacute studies were conducted using diisopropyl phosphorofluoridate (DFP), a known neurotoxic agent, at subcutaneous dosages of 1.0 mg/kg. In the acute studies, both DFP and trichlorfon markedly inhibited tissue cholinesterase activities but only DFP elicited a significant inhibition of NTE. In the subacute studies, DFP produced a characteristic central–peripheral distal axonopathy in the 18-day period of study which was confirmed by clinical and morphological evidence and by marked inhibition of neuronal NTE. Trichlorfon caused little or no obvious neurotoxicity, an observation that was supported by minimal morphological changes and impairment of walking ability and no inhibition of brain or spinal cord NTE.