A rapid, sensitive, and accurate high-performance liquid-chromatographic-mass spectrometric (HPLC-MS) method, with estazolam as internal standard, has been developed and validated for determination of aripiprazole in human plasma. After liquid-liquid extraction the compound was analyzed by HPLC on a C-18 column, with acetonitrile-30 mm ammonium acetate containing 0.1% formic acid, 58:42 (v/v), as mobile phase, coupled with electrospray ionization mass spectrometry (ESI-MS). The protonated analyte was quantified by selected-ion recording (SIR) with a quadrupole mass spectrometer in positive-ion mode. Calibration plots were linear over the concentration range 19.9-1119.6 ng mL(-1). Intra-day and inter-day precision (CV%) and accuracy (RE%) for quality-control samples (37.3, 124.4, and 622.0 ng mL(-1)) ranged between 2.5 and 9.0% and between 1.3 and 3.5%, respectively. Extraction recovery of aripiprazole from plasma was in the range 75.8-84.1%. The method enables rapid, sensitive, precise, and accurate measurement of the concentration of aripiprazole in human plasma.