A rapid method for the simultaneous determination of oxcarbazepine (OXC) and its active metabolite (10-hydroxycarbazepine) in human and rat plasma by reversed phase high-performance liquid chromatography is described. The method involves a simple one-step extraction of the drugs from plasma with dichloromethane. The extract was evaporated and the residue was reconstituted with mobile phase and injected onto a Novapak C18 column. The eluting solvent was 20% acetonitrile in water at a flow rate of 1.5 ml/min and the detector was monitored at 215 nm. The detection limit of OXC and 10-hydroxycarbazepine was 50 and 20 ng/ml, respectively. The within-day and between-day coefficients of variation for OXC and its active metabolite were 2.57-6.95% and 4.21-8.3%, respectively. The relative and absolute recoveries varied between 71.4% and 104.0%. The applicability of the analytical procedure to pharmacokinetic studies was illustrated.