The metabolic fate of fenitrothion, O,O-dimethyl-O-(3-methyl-4-nitro phenyl) phosphorothioate, was investigated in rat tissues during the first 24 hours following the intramuscular administration of the pesticide in the animal. High performance liquid chromatography (HPLC) of the pesticide and its metabolites formed in liver, kidney and brain showed the time-dependent sequential conversion of the pesticide into three major metabolites. These metabolites were separated and purified to homogeneity by HPLC and characterized by IR spectroscopy as O,O-dimethyl-O-)3-methyl-4-aminophenyl) phosphorothioate (metabolite I), O,O-dimethyl phosphorothioate (metabolite II), and O,O-dimethylphosphate (metabolite III). These results indicated reduction of the nitro group in fenitrothion aas the first step, followed by the hydrolytic cleavage of the P-O-aryl bond in metabolite I and the oxidative desulphurylation of metabolite II. Fenitrothion was found to induce ultrastructural changes in liver cells especially after 12 h exposure nuclear membrane was completely distorted, nuclear intactness was totally lost and smooth endoplasmic reticulum and Golgi apparatus were abnormally enlarged. In 24 h, however, the regeneration of the nuclear material was observed.