A simple, sensitive, and highly reliable liquid chromatographic method using fluorescence detection is described for the simultaneous determination of metoprolol and alpha-hydroxymetoprolol in plasma and urine. This method involves a single extraction of the compounds with the internal standard pindolol from alkalinized plasma or urine into dichloromethane. A reconstituted aliquot with a mobile phase is injected onto a reversed-phase, Zorbax ODS column, and the detection is achieved by the excitation and emission wavelengths at 230 and 300 nm, respectively. The assay is reproducible and precise for metoprolol and alpha-hydroxymetoprolol in both plasma and urine samples, as judged by a coefficient of variation of less than 9.2% at all concentrations examined. The standard curves for metoprolol and alpha-hydroxymetoprolol are linear over 10-200 ng/ml in plasma and over 0.5-10 micrograms/ml in urine. The lower detection limit is 2 ng/ml for each of the compounds in plasma using a 0.5-ml sample. Preliminary data on the oxidation polymorphism of metoprolol in Japanese subjects are reported using the current assay method. In 183 Japanese subjects no poor metabolizer of metoprolol has been identified so far.