OBJECTIVE To develop an HPLC method with direct injection for the simultaneous determination of metoprolol (M) and alpha-hydroxymetoprolol (HM) in human urine. METHODS Urine (200 microliters) was diluted with eluate and injected into the chromatograph. Samples were separated on an ODS column by isocratic binary elution and monitored by fluorescence detection. RESULTS No potential interfering peaks were identified. M and HM gave rapid elution and baseline resolution. The linear curves of both analytes ranged between 0.2 and 100 mg.L-1. The response sensitivity was approximately 0.1 mg.L-1 and the coefficients of variation in the assay were within 8% for both compounds. A typical application in oxidation phenotyping was presented for one healthy volunteer who received 100 mg of oral metoprolol. CONCLUSIONS The method can be used for the investigation of genetic polymorphism of metoprolol oxidation in the large populations.