&NA;Although fentanyl has been used widely as a short‐acting narcotic analgesic, its metabolism in humans has not been clarified. In this study, three fentanyl metabolites were identified in the urine of eight surgical patients receiving 0.3‐0.5 mg of fentanyl intravenously. The metabolites 4‐N‐(N‐propionylanilino)piperidine, 4‐N‐(N‐hydroxypropionylanilino)piperidine and l‐(2‐phenethyl)‐4‐N‐(N‐hydroxypropionylanilino)piperidine, and unchanged fentanyl were identified by GC‐mass spectrometry in urine collected 6 h after administration. Fentanyl and its main metabolite, 4‐N‐(N‐propio‐nylanilino)piperidine, were determined quantitatively in the urine of five additional patients receiving 0.5 mg fentanyl intravenously. Urinary excretion of fentanyl and 4‐N‐(N‐propionylanilino)‐piperidine during the first 12 h after injection accounted for 0.3‐4.0% and 26 to 55% of the dose, respectively.