A sensitive and specific method for the quantitative determination of morphine in human plasma is presented. Morphine was extracted from plasma by solvent extraction with ethyl acetate and derivatized to its heptafluorobutyrate derivative. The derivatives were measured by gas chromatography-negative ion chemical ionization mass spectrometry without any further purification. Using this detection mode, a diagnostic useful fragment ion at m/z 637 is obtained at high relative abundance. Deuterated morphine was used as an internal standard. Calibration graphs were linear within a range of 0.78 ng/ml and 50 ng/ml. Inter-assay precision was 2.3% (2.85 ng/ml) and 1.4% (14.25 ng/ml), intra-assay variability was found to be 1.5% (3.71 ng/ml) and 0.5% (14.54 ng/ml). Accuracy showed deviations of -9.3% (2.85 ng/ml) and -4.2% (14.25 ng/ml). The method is rugged and robust and has been applied to the batch analysis of morphine during pharmacokinetic profiling of the drug.