A reliable method has been proposed for the simultaneous determination of risperidone and its active metabolite 9-hydroxyrisperidone in plasma by dispersive liquid-liquid microextraction (DLLE) in combination with high-performance liquid chromatography-ultraviolet detector. All important variables influencing the extraction efficiency, such as pH, types of the extraction solvent and the disperser solvent and their volumes, ionic strength and centrifugation time were investigated and optimized. The method was validated over the concentration range 10-200 ng/ml. Mean recoveries were 96.8% for risperidone and 98.3% for 9-hydroxyrisperidone. Intra-and inter-day relative standard deviations were less than 11.5% for both compounds, while accuracy, expressed as percent error, ranged from 1.4 to 12%. The limit of quantitation was 3 ng/ml for both analytes. The method was successfully applied for the determination of risperidone and 9-hydroxyrisperidone in plasma.