A transplantable, metastasizing prostatic adenocarcinoma (Tumor I) in Lobund Wistar rats was examined for activity and distribution of five hydrolytic enzymes and for ability to accumulate radioactive zinc. The results suggest that the tumor had arisen in the ventral lobe of the prostate and that its growth was not affected by orchiectomy, adrenalectomy, or replacement treatment with exogenous androgen or corticosteroids. The androgen independency of the tumor was further shown by the low uptake of 3H-testosterone, in contrast to the high uptake in the ventral prostate. Tumor growth was retarded by Cytoxan but not by 5-fluorouracil, Estracyt, or streptozotocin, three agents clinically effective in the treatment of some patients with prostatic cancer resistant to endocrine therapy. It is concluded that this tumor in Lobund Wistar rats may be an adequate model for human prostatic cancers resistant to the agents mentioned above.