An gas chromatography-electron-capture detection method has been developed for simultaneous determination of fluoxetine and p-trifluoromethylphenol (TFMP), an O-dealkylated metabolite of fluoxetine in human liver microsomes. Prior to the analysis, aliquots of alkalinized microsomal mixture were extracted with ethyl acetate solvent containing acetonitrile (10%, v/v) and the derivatizing reagent, pentafluorobenzenesulfonyl chloride (0.1%, v/v). The organ phase was retained and taken to dryness, the residue was reconstituted in methanol, and the aliquot of extracts was injected directly into a gas chromatograph equipped with an electron-capture detector. 2, 4 Dichlorophenol was added to the initial incubation mixture and carried through the procedure as the internal standard. The method provided the mean recoveries of up to 103% for fluoxetine and 104% for TFMP. Acceptable relative standard deviations were found for both within-run and day-to-day assays. The practical limit of detection (signal-to-noise ratio53) was 1.62ng/ml for TFMP and 6.92ng/ml for fluoxetine in human liver microsomes, and the limit of quantitation was 8.1 pg for TFMP and 34.6 pg for fluoxetine. The assay is rapid and sensitive and has been applied successfully to simultaneous quantification of fluoxetine and TFMP in human liver microsomes with different CYP2C19 genotypes.