A method for the measurement of the beta-blocker timolol and its 13C3-labeled analog in human plasma is described. A known amount of an internal standard, [2H9]timolol, is added to each plasma sample. The method employs a reversed-phase C18 cartridge for isolation of the drug-containing fraction, capillary column GLC of the trimethylsilyl derivatives, and detection via selected-ion monitoring. The ions monitored for quantification (m/e 86 from timolol and m/e 89 and 95 from the 13C3- and 2H9-labeled analogs, respectively) arise from the side chains of the three compounds [e.g., CH2N+HC(CH3)3 for m/e 86]. Plasma levels of timolol and [13C3]timolol in human subjects given 5 or 10 mg of each compound were followed simultaneously for 12 hr postdosing. A detection level of 0.5 ng/ml of plasma was found. No evidence was found for a metabolic kinetic isotope effect since the ratios of the two species of drug in the plasma samples were the same as the ratios in the administered dose.