The pathogenesis of organophosphate ester induced delayed neuropathic effects was investigated in chickens treated with phenyl- saligenin-phosphate (4081236) (PSP) or with tri-ortho-tolyl- phosphate (78308) (TOTP). Adult White-Leghorn-chickens were administered intramuscular injections of 2, 3, or 10mg/kg PSP, or were orally administered 360 or 500mg/kg TOTP. Ataxia and weakness were observed after exposure to either compound. Neuropathy target esterase was decreased by more than 85 percent by 24 hours after exposure, but this returned to control values after 2 weeks. Electron microscopy revealed qualitatively similar lesions secondary to PSP and TOTP administration. Injuries of the distal regions of large myelinated axons in long spinal cord tracts and peripheral nerves were observed. The peripheral nerve lesions were characterized by intraaxonal collections of mitochondria, dense and lamellar bodies, and granular degeneration of neurofilaments. These lesions progressed to Wallerian like degeneration. The authors conclude that there is a significant correlation between the percentage of injured nerve fibers and the severity of clinical symptoms.