A gas chromatographic-mass spectrometric (GC/MS) method for the determination of β-{3-oxo-s-triazolo[4, 3a]pyridin-3(2H)-one}-propionic acid (TPA), a metabolite of trazodone hydrochloride, in biological fluids has been developed. Deuterated analogue of TPA was used as an internal standard. TPA was extracted with ethyl acetate from plasma and urine, and derivatized into a pentafluorobenzyl ester. The reaction mixture was further purified by high-performance liquid chromatography on a reversed-phase column. The eluent containing pentafluorobenzyl ester of TPA and deuterated TPA was collected, and extracted with benzene. GC/MS was carried out by monitoring the molecular ions. The determination limit was 500 pg/ml, and the intra and inter-assay coefficients of variation for the determination of TPA in plasma were 6.7 ?? 9.4% and 7.3 ?? 11.5%, respectively. Plasma levels and urinary excretion were determined by the present method for five healthy volunteers who had received a 50 mg oral dose of trazodone hydrochloride.