A high-performance liquid chromatographic method is presented for the determination of metoprolol in human plasma. Metoprolol was extracted from plasma by a single extraction procedure with 4-methylpropranolol as the internal standard. Chromatography was done on a reversed-phase column with fluorescence detection. The minimum detectable concentration was 5.0 ng/ml of plasma. The standard curve was linear in the range evaluated, 10-300 ng/ml. The within-run coefficient of variation was 2.3-6.0%, and the day-to-day variation was 6.8%. The method is free from interference by major metoprolol metabolites.