Thirty min before prostatectomy, either 400 μCi [1,2-3H] testosterone (51 Ci/mM) or 400 μCi [1,2-3H] 5α-dihydrotestosterone (44 Ci/mM) was injected intravenously into human patients with benign prostatic hypertrophy (bph). Total radioactivity was counted in aliquots of blood, of skeletal muscle, of prostate and of different parts of the prostatic adenomas. After extraction, the unconjugated steroids were isolated by TLC in different systems. The following results were obtained: 1. The endogenous plasma concentrations of testosterone and 5α-dihydrotestosterone (5α-DHT) in patients with bph are within the normal range. 2. Testosterone and 5α-DHT accumulate to a greater extent in prostatic tissue than in muscular tissue. 3. 5α-DHT is the main metabolite in prostatic tissue, the extent of 5α-reduction depending on the predominant type of tissue present. 4. In muscle no 5α-reduction of testosterone occurs, whereas exogenous 5α-DHT is efficiently metabolised to androstanediols and androsterone. 5. The androgen concentrations and the distribution of metabolites in the blood do not reflect the situation in the prostate or in skeletal muscle.