The disposition, metabolism, and pharmacokinetics of the plasticizer tri-o-cresyl-phosphate (78308) (TOCP) in plasma and some tissues of the hen were investigated. Laying hens were given a single oral dose of 50mg/kg carbon-14 labeled TOCP and sacrificed 0.5, 1, 2, or 5 days later. In plasma the half live of the carbon-14 labeled TOCP was 2 days. High performance liquid chromatography and liquid scintillation counting were used to analyze TOCP and its metabolites in the plasma, liver, kidneys, and lungs. Between 0.5 and 1 day after dosing, TOCP peaked in concentration in the plasma. Disappearance of TOCP from the plasma followed monoexponential kinetics with a half life of 2.2 days. At all time points there were appreciable concentrations of saligenin-cyclic-o-tolyl- phosphate in the plasma, the liver, kidneys, and lungs, registering half lives of 2.06, 1.36, 1.11, and 4.44, days in each sample type, respectively. The authors suggest that the presence of this active neurotoxic metabolite may contribute to the increased sensitivity of the hen over the rat to organophosphorus induced delayed neurotoxicity caused by TOCP exposure.