Abstract: An HPLC method for the determination of dextrorphan, an active metabolite of dextromethorphan, in plasma was established using column switching technique. The column switching system was equipped with a per-column of 30 mm×5 mm ID, packed with μ Bondapak C₁₈, 37～50 μn, and an analytical column of 150 mm×5 mm ID, packed with YWG-C₁₈, 5 μm. A 0.2 % acetic acid solution was used as the pretreating mobile phase to wash out impurities from the per-column. The analytical mobile phase consisted of acetonitrile—water—acetic acid—triethyl-amine—dichloromethane (17:82:1:0.05:0.025). The plasma samples were directly injected into the HPLC system after enzymatic hydrolysis of dextrorphan glucuronide ester conjugate to free form with β-glucuronidase. The dextrorphan was monitored with a fluorescence detector at 290 nm (excitation) and 315 nm (emission). The method was linear within the plasma concentration range of 20～640 ng/ml (r=0.9987), and the detection limit was 4 ng/ml. The mean recoveries of the method averaged 103.8%. The relative standard deviations of the assay were less than 10% for both withinday and between-days.