BACKGROUND Measurement of biogenic amines and their metabolites in plasma has been used for decades to explore the pathophysiology of neuropsychiatric disorders or to examine the mode of action of psychotropic medications. Many advantages of ultrafiltration and high-performance liquid chromatography (HPLC) in assaying human plasma content have been described in our previous study. METHODS The present HPLC assay, applying a dual-channel electrochemical detector, provides an additional reliable measurement or confirmation of peaks by identifying each peak retention capacity factor and its current ratio in the dual-channel detector. Simultaneous measurements of plasma serotonin, catecholamines and their metabolites by ultrafiltration and microbore HPLC technique is applied to evaluate moclobemide and fluoxetine therapy in patients with major depression. RESULTS This present HPLC method provides highly specific detection, free from interference by the large offscale peak of plasma ultrafiltrates. Isocratic separation of all analytes by a microbore column is achieved within 15 mins. The detection limit (signal-to-noise ratio = 3) of this method is about 0.2-0.5 pg per injection for all analytes. In addition, the required volume of plasma samples is only 100 microliters. CONCLUSIONS This rapid, simple and sensitive method for the measurement of human plasma serotonin, catecholamines and their metabolites can be used as a clinical or biomedical research tool. Blood loss is minimal in the present assay, especially in repeated blood sampling for pharmacokinetic studies.