The measurement of nanogram range of 1-butyryl-4-cinnamylpiperazine (BCP) in the human urine in the presence of its metabolites was successfully performed with stable isotopes and gas chromatograph-mass spectrometer equipped with a multiple ion detector (GC-MS-MID). From the urinary excretion data, the elimination kinetics of BCP was determined. There did not exist any appreciable isotope effect in the urinary excretion kinetics of BCP following oral administration of an equimolar mixture of BCP-d₀ and BCP-d₅ to a human subject, indicating pharmacokinetic equivalence of the unlabeled and labeled BCP. The technique we employed in this study should demonstrate the general usefulness in determining pharmacokinetic parameters of a drug.