The effects of 5α-androstane-3,17-dione2), of 5α-androstane-3β,17β-diol and of 5α-androstane-3α,17β-diol on protein and nucleic acid content as well as on the activities of some enzymes have been studied in the ventral prostate and the seminal vesicles of immature castrated rats. The androgens were administered in doses of 0.1 mg or 1 mg three times at 24 h intervals respectively, and the animals were sacrificed 24 h after the last injection. In accordance with the literature 5α-androstane-3,17-dione had a distinctly greater effect on the ventral prostates than on the seminal vesicles. Furthermore it could be demonstrated that in seminal vesicles this androgen had no effect on cell proliferation, whereas cell metabolism was slightly stimulated. The lack of cell proliferation in seminal vesicles is probably due to a smaller conversion of the steroid to 5α-dihydrotestosterone, i.e. to a weaker activity of the 17β-oxidoreductase in this target organ. The 5α-androstane-3β,17β-diol as well as the 5α-androstane-3α,17β-diol had a marked effect on DNA increase, comparable to that of 5α-dihydrotestosterone in both target organs. In the case of the 5α-androstane-3α,17β-diol this effect could be due to its high conversion rate to 5α-dihydrotestosterone. However, this is not the case for the 5α-androstane-3β,17β-diol. It seems possible that the 3β-hydroxy group of an androgen also exerts a cell proliferative effect. Concerning stimulation of cell metabolism in both target organs a greater effect was found after the administration of 5α-androstane-3α,17β-diol. This again leads to the conclusion that there are different sites of androgen action at the cellular level.