SUMMARY: The metabolism of testosterone in vitro by normal benign hyperplastic and carcinomatous prostatic tissues has been compared. 5α-Dihydrotestosterone was the predominant metabolite followed by 3α- and 3β-androstanediol. No difference in the pattern of metabolism, either quantitatively or qualitatively, was found in normal, benign hyperplastic and welldifferentiated carcinomatous prostatic tissue. In poorly differentiated carcinomatous tissue there was less formation of all 5α-reduced metabolites. Treatment of patients with carcinomata with diethylstilboestrol resulted in greatly decreased activity of prostatic 5α-reductase with a considerable proportional increase in 17β-dehydrogenase activity. The addition of oestradiol-17β or stilboestrol in vitro also inhibited the formation of 5αdihydrotestosterone and androstanediols by all types of prostatic tissue, but only when high concentrations of oestrogen were used and there was no increase in 17β-dehydrogenase under these conditions. In contrast, progesterone was a much more effective inhibitor of 5α-reductase in vitro than was oestrogen.