A rapid, highly sensitive method for the determination of morphine and its metabolites morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G) and normorphine has been developed using high-performance liquid chromatography-electrospray mass spectrometry, with the deuterated analogues as internal standards. The analytes were extracted automatically using end-capped C2 solid-phase extraction cartridges. Baseline separation of morphine, M3G and M6G was achieved on a LiChrospher 100 RP-18 end-capped analytical column (125x3 mm I.D., 5 microm particle size) with water-acetonitrile-tetrahydrofuran-formic acid (100:1:1:0.1, v/v) as the mobile phase. Morphine and normorphine coeluate and were separated mass spectrometrically. The mass spectrometer was operated in the selected-ion monitoring mode using m/z 272 for normorphine, m/z 286 for morphine, m/z 462 for morphine-6-glucuronide. Due to an interfering peak, M3G was measured by tandem mass spectrometry in the daughter-ion mode. The limits of quantitation achieved with this method were 1.3 pmol/ml for morphine, 1.5 pmol/ml for normorphine, 1.0 pmol/ml for M6G and 5.4 pmol/ml for M3G in serum or cerebrospinal fluid. The limits of quantitation achieved in urine were 10 pmol/ml for morphine, 20 pmol/ml for normorphine and M6G and 50 pmol/ml for M3G using a sample size of 100 microl. The method described was successfully applied to the determination of morphine and its metabolites in human serum, cerebrospinal fluid and urine in pharmacokinetic and drug interaction studies.