A fast and sensitive method to quantify oxcarbazepine (OXC) and its active metabolite, 10,11-dihydro-10-hydroxycarbamazepine (MHD) in human plasma using HPLC-MS/MS has been developed. The method involved liquid-liquid extraction (LLE), with diethyl ether-diclhoromethane (60:40v/v) using deuterade carbamazepine (d10-carbamazepine) as internal standard (IS). The analytes and IS were separated using an isocratic mobile phase (acetonitrile/water (50:50v/v)+20 mM acetic acid) on the analytical column Phenomenex Luna C18 5 microm (150 mm x 4.6 mm) at room temperature. Detection was performed by a Micromass Quatro LC mass spectrometer in the reaction monitoring mode using positive electrospray ionization (ESI+). The MS-MS ion transition monitored were m/z 253>208 for OXC, m/z 255>194 for MHD and m/z 247>204 for IS. Over the range 20-5250 ng/ml for OXC and 40-10,500 ng/ml for MHD, the calibration curves were defined by the following equations: y = 0.00568 + 0.00296x -5.70e - 8x(2) and y = 0.00749 + 0.00178x - 5.70e - 8x(2) for OXC and MHD, respectively. All coefficient of determination (r(2)) were close to unity (0.9986-0.9994). The lower limits of quantification obtained as a result of the LLE procedure was 20 ng/ml for OXC and 40 ng/ml for MHD. The statistical evaluation of the developed method was conducted by examining within-batch and between-batch precision data, which were within the required limits. The suitability of the assay for pharmacokinetics studies was determined by measuring OXC and MHD concentration after administration of a single 10 ml of OXC oral suspension (6%) in plasma human of healthy volunteers.