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A gas chromatographic/mass spectrometric method for the specific determination of oxprenolol and 2H6-labelled oxprenolol when both are present in the same sample is described. After addition of 13C3-labelled oxprenolol as internal standard, plasma is alkalized and extracted by a mixture of dichloromethane and diethyl ether. The residue following evaporation of the organic phase is derivatized with heptafluorobutyric anhydride. Negative ion detection with N2O as reagent gas is used for the measurements at m/z 488, 491 and 494 for oxprenolol, the 13C3-labelled internal standard and 2H6-labelled oxprenolol, respectively. The precision and accuracy of the analytical method were investigated using samples containing both unlabelled and 2H6-labelled oxprenolol. The overall mean recovery (% +/- SD, n = 70) in the concentration range 20-1500 nmol l-1 (around 6-450 ng ml-1 of the hydrochloride salts) was 100.6 +/- 3.3 and 101.0 +/- 3.5 for oxprenolol and 2H6-labelled oxprenolol, respectively. The limit of quantification was around 20 nmol l-1 for both compounds.