A "hen model" or organophosphorus induced delayed neuropathy (OPIDN) has been developed using white leghorn exposed acutely to one of five dosages of tri-ortho-cresyl phosphate (TOCP), between 300 to 700 mg/Kg. Neuropathy target esterase was studied in brain and peripheral nerve 24 and 48 hrs following exposure. Behavioral symptoms abnormality was assessed from days 1 through 20 after exposure using a 7 point rating scale and neuropathological examination was conducted on sample collected from animals on days 0, 7, 14 and 21. Neuropathological abnormalities were indicated by damage scores between 0 (no damage) and 4 (gliosis of brain tissue, myelin loss, appearance of axonal foci etc and more than 55% degeneration of peripheral nerve fibres). TOCP (600 and 700 mg/Kg, orally) was able to inhibit NTE more than 75% in brain and peripheral nerves. TOCP at the same dosage was also capable of resulting maximal levels of neuropathological score at 4. After exposure to doses weight loss was observed abruptly in a greater extent at the beginning leading to a change in weight gain till the end of the experiment. Behavioral signs were also dose dependent. Symptoms (gain abnormality, ataxia, paresis) were noted on the early stage of experiment. Inhibition of NTE was 65% could not be reached in hens given TOCP without causing lethality and no significant ataxia or lesions developed in those birds. Behavioral signs were also observed to be late onset. These data indicate that more than 75% inhibition of peripheral nerve NTE after 24 hr exposure was predictive of severe behavioral abnormalities and pathology in the hen whereas less peripheral NTE inhibition was indicative of less severe behavioral abnormalities and a lower score for neuropathological damage.