This study was undertaken to investigate whether depletion of hepatic nonprotein sulfhydryls (NPSH) by acetaminophen would potentiate the toxicity of organophosphates which are detoxified by glutathione transferases. Acetaminophen caused a dose dependent decrease of NPSH in mouse liver. At the dose of 600 mg/kg, which decreased hepatic NPSH by 90%, acetaminophen did not potentiate the effects of the organophosphorus insecticides methylchlorpyrifos, methylparathion or dichlorovos on esterases. On the other hand, depletion of hepatic NPSH by diethylmaleate increased the toxicities of the insecticides. While the reported inhibition of mixed function oxidase activity by acetaminophen may explain the lack of potentiation of methylchlorpyrifos and methylparathion which need to be converted to their oxygen analogs, its failure in potentiating the toxicity of dichlorvos, which does not require metabolic activation, suggests that other mechanisms are involved. The finding that acetaminophen decreased NPSH only in the liver and, to a minor extent, in the kidney, while diethylmaleate caused significant depletion of NPSH in several tissues, suggests that extrahepatic glutathione may be relevant to the detoxication of certain organophosphates.