The effect of adrenalectomy (Adx), SKF 525-A, phenobarbital (PB), and diethyl maleate (DEM) on the acute toxicity of fenitrothion was investigated in male rats by assessing the degree of plasma cholinesterase activity. PB, 60 mg/kg/day for 3 days, exerted no protective effect on the toxicity of fenitrothion (100 mg/kg, p.o.) given 24 h after the last injection. In adrenalectomized and SKF 525-A-pretreated rats, the toxicity of fenitrothion was lower than that of the controls. Fenitrothion toxicity was increased by administration of DEM (1 ml/kg), which depletes hepatic glutathione (GSH) levels. In vitro, the rates of fenitrothion decomposition and fenitrooxon formation by microsomes were markedly affected by PB, SKF 525-A and Adx. The decomposition of fenitrooxon by the microsomal fraction and GSH-dependent decomposition of fenitrooxon by the soluble fraction were not affected by PB, SKF 525-A and Adx pretreatment. The GSH-dependent decomposition of fenitrothion and fenitrooxon was increased by addition of GSH to the incubation mixture. The present results indicate that the GSH-dependent metabolic pathway plays an important role in the detoxication of fenitrothion.