This article reports an isotope dilution mass spectrometric method for the simultaneous measurement of testosterone (T), dihydrotestosterone (DHT), and 5 alpha-androstan-3 alpha, 17 beta-diol (3 alpha-diol) in human plasma and prostatic tissue. After addition of tri-deuterated steroids as internal standards to prostatic tissue homogenates or plasma samples, extraction was performed with diethylether. The extracts were purified by two chromatographic steps (Sep-Pak C 18 cartridge and Sephadex LH-20) and injected into a gas chromatograph coupled with a mass spectrometer after derivatization with heptafluorobutyric acid. This method was highly specific and showed good precision, accuracy, reproducibility and sensitivity. T, DHT, and 3 alpha-diol were measured in human plasma and in prostatic tissue of seven patients with benign prostatic hyperplasia (BPH) treated for 3 months with a long acting GnRH analog before surgery. Plasma levels of T, DHT, and 3 alpha-diol were reduced by GnRH analog treatment to castrate levels. The tissue concentrations of the same steroids, compared with those obtained from 19 untreated patients, showed a mean reduction of about 90% for DHT and 3 alpha-diol, and about 75% for T. These results suggest that about 90% of prostatic DHT and 3 alpha-diol depend on testicular activity because they are dramatically reduced after pharmacologic castration.