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Benign prostatic hyperplasia and prostate cancer can be treated with the alpha-reductase inhibitors, finasteride and dutasteride, when pharmacodynamic biomarkers are useful in assessing response. A novel method was developed to measure the substrates and products of 5 alpha-reductases (testosterone, 5 alpha-dihydrotestosterone (DHT), androstenedione) and finasteride and dutasteride simultaneously by liquid chromatography tandem mass spectrometry, using an ABSciex QTRAP (R) 5500, with a Waters Acquity(TM) UPLC. Analytes were extracted from serum (500 mu L) via solid-phase extraction (Oasis (R) HLB), with C-13(3)-labelled androgens and d9-finasteride included as internal standards. Analytes were separated on a Kinetex C18 column (150 x 3 mm, 2.6 mu m), using a gradient run of 19 mm. Temporal resolution of analytes from naturally occurring isomers and mass +2 isotopomers was ensured. Protonated molecular ions were detected in atmospheric pressure chemical ionisation mode and source conditions optimised for DHT, the least abundant analyte. Multiple reaction monitoring was performed as follows: testosterone (m/z 289 -> 97), DHT (m/z 291 -> 255), androstenedione (m/z 287 -> 97), dutasteride (m/z 529 -> 461), finasteride (m/z 373 -> 317). Validation parameters (intra- and inter-assay precision and accuracy, linearity, limits of quantitation) were within acceptable ranges and biological extracts were stable for 28 days. Finally the method was employed in men treated with finasteride or dutasteride; levels of DHT were lowered by both drugs and furthermore the substrate concentrations increased. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).