Abstract: AimTo develop an HPLC-MS assay for determination of finasteride in human plasma and to investigate the bioequivalence in healthy volunteers. MethodsAfter alkalization with sodium hydroxide, plasma was extracted with ethyl acetate and separated using a C₁₈ column with a mobile phase of methanol-water (85∶15). LC-ESI-MS was performed in the selected ion monitoring (SIM) mode using target ions at m/z 395 for finasteride and m/z 407 for the IS. The fragmentor voltage was 120 V. A randomized crossover design was performed in 20 healthy volunteers. In the two study periods, a single 10 mg dose of each tablet was administered to each volunteer. ResultsCalibration curves were linear over the range 1-200　μg·L^-1 (R=0.998 6). The limit of determination for finasteride in plasma was 0.05 μg·L^-1. The recovery of finasteride from plasma was in the range of 85.9%-98.7%. The results of variance analysis and two one-side T-test showed that there was no significant difference between the two formulations in the AUC and C_max. Conclusion The assay was proved to be sensitive, accurate and convenient. The two formulations were bioequivalent.