Chickens which developed organophosphorus ester-induced delayed neuropathy (OPIDN) due to a single oral dose of 360 mg/kg tri-ortho-tolyl phosphate were followed for up to 64 days following toxicant administration. Neuropathy was well developed by day 14. Progressive, marked but incomplete clinical improvement was observed between that time and day 49, associated with regenerative process was initially noted on day 16 at a distal, non-terminal level of the tibial nerve branch to the lateral head of the gastrocnemius muscle. One or more axonal sprouts were seen in bands of Büngner, but only one regenerated fiber per band myelinated and grew. By day 64 the nerves from treated hens closely resembled those from the controls. Myelinated nerve fiber degeneration was noted in distal regions of long spinal cord white matter tracts, but no subsequent regeneration was observed in this region. These studies indicate that in OPIDN there is a transient period of neuronal injury, and that damaged cells having peripherally directed neurites are likely able to undertake axonal regeneration.