Fenitrothion (FT) at low concentration of around 10^-6M reacted with mouse liver homogenate and disappeared immediately similarly to fenitrooxon (FO). This occurred mostly in the soluble fraction of mouse liver and was dependent on glutathione (GSH). The main metabolite detected was desmethyl-FT in the case of the reaction with a small amount of mouse liver homogenate, while desmethyl-FO was predominant metabolite in the reaction with relatively large amount of homogenate. In both cases all desmethylated metabolites were obtained more than 50% of added FT. These facts reveal that at such low concentration, FT is detoxified mainly through desmethylation reaction catalyzed by GSH S-transferase (GTase). On the other hand the amount of 4-nitro-m-cresol was less than 10%, so the oxidative dearylation by mixed function oxidase (MFO) contributed little to the detoxication of FT at low concentration. The Km and Vmax values of both GTase and MFO are well consistent with this reaction mechanism.