Three different dutasteride extraction methods were compared and a method based on liquid-liquid extraction (LLE) using methyl tert-butyl ether and methylene chloride was proved to be more effective than others for the extraction of dutasteride and finasteride, the internal standard (IS), from rat plasma. Additionally, a method composed of the LLE extraction, liquid chromatography, and multiple reaction monitoring (MRM) to target dutasteride and IS was validated by assessing specificity, linearity (r 2 = 0.9993, 5 -400 ng/mL), sensitivity (the limit of detection: 4.03 ng/mL; the limit of quantitation: 12.10 ng/mL), accuracy (intra-day: 89.4 -105.9%; inter-day: 84.9 -100.9%), precision (intra-day: 0.8 -6.9%; inter-day: 2.9 -15.9%), and recovery (84.7 -107.8%). Since the validated method was successfully applied to a pharmacokinetic study of dutasteride, it can be useful for the pharmacokinetic evaluation of newly developed dutasteride formulations.