This study reports three toxicologic effects of S,S,S-tributyl phosphorotrithioite (merphos) in hens: acute cholinergic, late acute, or delayed neurotoxic. These effects can be differentiated by route of administration, dosage required, severity and reversibility of clinical signs, and production of pathologic lesions of the nervous system. Oral administration of a single dose (200 to 2000 mg/kg) of merphos caused weak cholinergic effects, which were rapidly relieved by atropine sulfate treatment. Four days after start of administration of large daily doses (80 mg/kg/day) a late acute effect was observed. The clinical signs of the late acute effects were identical to those produced by n-butyl mercaptan ( nBM), a hydrolytic product of merphos, and were not relieved by atropine sulfate. The late acute effect overlapped with the clinical signs of delayed neurotoxicity. Late acute toxicity was not seen with topical application of single or daily doses of merphos; rather, delayed neurotoxicity was consistently produced. Degeneration of the anterior columns in the spinal cord was identical to that found in tri- o-cresyl phosphate-treated hens and was the most consistent histopathologic change. Topical administration of merphos caused a more prolonged inhibition of plasma butyrylcholinesterase activity than the orally administered compound. Orally administered merphos was rapidly metabolized in the gastrointestinal tract directly to nBM or following its oxidation to S,S,S-tributyl phosphorotrithioate. nBM apparently caused the late acute toxic effect. Topically administered merphos, which was not metabolized in the gastrointestinal tract, caused delayed neurotoxicity but did not produce a late acute effect. The present results demonstrate that evaluation of the neurotoxic effect of organophosphorus esters should include investigation with both oral and topical application.