This study was undertaken to determine if metabolites of fentanyl might be useful in the detection and monitoring of substance abuse. The presence of fentanyl and two of its metabolites in the urine and saliva of seven female patients receiving small doses (110 +/- 56 micrograms) of fentanyl was studied up to 96 h from the time of administration. Fentanyl and its two metabolites (norfentanyl and despropionylfentanyl) were extracted from samples and analyzed by gas chromatography/mass spectrometry. Unchanged fentanyl was detectable in urine in all patients immediately postoperatively and in 3 of 7 patients at 24 h. By 72 h, fentanyl was undetectable. Norfentanyl was present in larger quantities than fentanyl immediately postoperatively and was detected in all patients at 48 h and in 4 of 7 patients at 96 h. Despropionylfentanyl was not detected in any of the urine specimens tested. Neither fentanyl nor its metabolites could be detected consistently at any time in saliva. Saliva testing does not appear to be a viable alternative to urine testing based on this study. Urinary norfentanyl might be considered as the substance of choice when testing for fentanyl abuse.