A high-performance liquid chromatographic method for the quantitation of finasteride in human plasma is presented. The method is based on liquid-liquid extraction with hexane-isoamylalcohol (98:2, v/v) and reversed-phase chromatography with spectrophotometric detection at 210 nm. The mobile phase consists of acetonitrile-15 mM potassium dihydrogenphosphate (40:60, v/v). Clobazam is used as the internal standard. The limit of quantitation is 4 ng/ml and the calibration curve is linear up to 300 ng/ml. Within-day and between-day precision expressed by relative standard deviation is less than 5% and inaccuracy does not exceed 8%. The assay was used for pharmacokinetic studies.