A sensitive, selective and reproducible high-performance liquid chromatographic assay for the simultaneous determination of metoprolol and alpha-hydroxymetoprolol in plasma was developed with (+/-)-ethyl-2-(4-(3-isopropyl-amino-2-hydroxy-propoxy)phenyl)-ethyl carbamate as the internal standard. Samples were acidified and subjected to an organic wash to remove interfering neutral and acidic components. Final extraction is made from the alkalinized aqueous phase with methylene chloride. The samples were chromatographed on a microparticulate silica gel column with fluorescent detection. Sensitivity was 3 ng/ml for metoprolol and 12.5 ng/ml for alpha-hydroxymetoprolol. Specificity was established by comparison of retention times of samples with standard materials. A typical absorption/disposition profile for metoprolol and alpha-hydroxymetoprolol is presented for one subject who received 50 mg of metoprolol tartrate (Lopressor).