Carcinoma in the dorsal prostate of Nb rats was produced by prolonged treatment with androgen or androgen plus estrogen. The most effective combination was androgen followed by estrogen, resulting in an incidence of 50% in 10 rats. In such a case, the rats did not show evidence of androgenization. Transplants were usually not sensitive to hormones, but an estrogen-dependent tumor (Nb-2Pr-E) was successfully transplanted in estrogenized hosts. Cells of estrogen-dependent tumors did not grow in males but remained dormant and could be stimulated to grow after many months by estrogen treatment. The resulting tumors remained estrogen dependent, with one exception which was autonomous. Tumors regressed following estrogen removal but eventually showed a spontaneous regrowth which was then found to be autonomous. In the case of growing tumors, the reduction of estrogen to subthreshold doses for maximal growth allowed stationary or only slow growth of the tumors, but these remained estrogen dependent. Estrogen-dependent tumors responded to tamoxifen treatment by a cessation of tumor growth or regression despite continued estrogen treatment. Tumor regrowth eventually took place after prolonged tamoxifen treatment, but the tumors remained estrogen dependent when transplanted.