A new, simple, specific, and rapid ultra high performance liquid chromatography method was developed and validated for the determination of aripiprazole and its active metabolite dehydroaripiprazole in rat plasma. The analysis was performed on a Zorbax Eclipse Plus C18 (50 mm x 2.1 mm, 1.8 mu m particles) column using gradient elution with a mobile phase consisting of acetonitrile and acetate buffer (30 mM, pH 5). Plasma samples were analyzed after a simple, one-step protein precipitation with acetonitrile. The method was validated and the specificity, linearity, limit of detection, limit of quantitation, precision, accuracy, recoveries, matrix effect, stability and robustness were determined. The analytes and internal standard were separated in 3.5 min and the total analysis time including the clean-up step was 8 min. Limit of detection was 0.012 mu g mL(-1) and 0.009 mu g mL(-1) for aripiprazole and dehydroaripiprazole, respectively. Limit of quantitation was 0.039 mu g mL(-1) and 0.029 mu g mL(-1) for aripiprazole and dehydroaripiprazole, respectively.