1. Sex differences observed in the metabolism of pentachlorothioanisole in rat were due to: (1) greater excretion in urine by females, and greater biliary excretion by males; (2) formation of pentachlorophenyl mercapturic acid pathway metabolites by females; and (3) redox-cycling between methylthio and methylsulphoxyl oxidation congeners in intermediary metabolites by females. 2. Three methylthio-turnover processes are proposed in the intermediary metabolism of pentachlorothioanisole.