Tritiated 5α-androstane-3α,17β-diol (3α-diol) and 5α-androstane-3β,17β-diol (3β-diol) respectively were administered to patients with benign prostatic hypertrophy (bph) undergoing prostatectomy. In prostate and skeletal muscle homogenates and in plasma the total radioactivity content as well as the formation of metabolites were measured. Histological examination of each ectomized prostate was performed to evaluate the cellular composition of the tissue. After 3α-diol injection, a higher uptake of radioactivity in the prostate was obtained than after 3β-diol. Within 30 min the 3α-isomer was very efficiently converted to 5α-DHT, while most of the 3β-isomer remained unchanged. There was, however, also after administration of the 3β-diol a substantial biconversion to 5α-DHT as has been confirmed by recrystallization to constant specific radioactivity. Only after 3β-diol epiandrosterone was detected in small but significant amounts. 3α-diol administration resulted in distinct concentrations of 3β-diol, whereas the conversion of 3β-diol to the 3α-isomer was insignificant. When comparing the histological composition of the prostatic tissue with the accumulation of radioactivity and the formation of metabolites only a weak correlation between glandular structure and radioactivity uptake after 3α-diol administration could be revealed.