This study describes a new simultaneous determination of fluvoxamine and fluvoxamino acid by automated column-switching high-performance liquid chromatography. The test compounds were extracted from 1.5 ml of plasma using chloroform-toluene (15:85, v/v), and the extract was injected into a hydrophilic metaacrylate polymer column for clean-up and a C18 analytical column for separation. The mobile phase for separation consisted of phosphate buffer (0.02 M, pH 4.6), acetonitrile and perchloric acid (60%) (62.4:37.5:0.1, v/v/v) and was delivered at a flow rate of 0.6 ml/min. The peak was detected using a UV detector set at 254 nm. The method was validated for the concentration range 0.8-153.6 ng/ml for fluvoxamine and 0.6-115.2 ng/ml for fluvoxamino acid, and their good linearity (r > 0.998) were confirmed. Intra-day coefficient variations (CVs) for fluvoxamine and fluvoxamino acid were less than 6.6 and 6.0%, respectively. Inter-day CVs for corresponding compounds were 6.3 and 6.5%, respectively. Relative errors ranged from -18 to 9% and mean recoveries were 96-100%. The limit of quantification was 1.2 and 0.9 ng/ml for fluvoxamine and fluvoxamino acid, respectively. This method shows successful application for pharmacokinetic studies and therapeutic drug monitoring.