A rapid, specific, and sensitive method utilizing reversed-phase ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) was developed and validated to determine finasteride levels in human plasma. The plasma samples were prepared by liquid-liquid extraction with ethyl acetate, evaporation, and reconstitution. MS/MS analyses were performed on a triple-quadrupole tandem mass spectrometer by monitoring protonated parent-->daughter ion pairs at m/z 373-->305 for finasteride and m/z 237-->194 for carbamazepine (internal standard, IS). The method was validated with respect to linearity, recovery, specificity, accuracy, precision, and stability. The method exhibited a linear response from 0.1 to 30 ng/mL (r(2)>0.998). The limit of quantitation for finasteride in plasma was 0.1 ng/mL. The relative standard deviation (RSD) of intra- and inter-day measurements was less than 15% and the method was accurate within -6.0% to 2.31% at all quality-control levels. The mean extraction recovery was higher than 83% for finasteride and 84% for the IS. Plasma samples containing finasteride were stable under the three sets of conditions tested and the processed samples were stable up to 29 h in an autosampler at 5 degrees C. Detection and quantitation of both analytes within 3 min make this method suitable for high-throughput analyses. The method was successfully applied to a pharmacokinetic study of finasteride in healthy volunteers following oral administration.