Nb rat prostatic adenocarcinomas have been successfully transplanted into groups of syngeneic Nb rats, as well as into congenitally athymic (nude) mice, with a high frequency of success. This rate of success is one characteristic of this system that allows for experimentation in vivo in large numbers of animals for statistical considerations such as the following: similarity to human prostatic adenocarcinoma, histopathologic response to appropriate hormonal therapy in the case of androgen-dependent tumors, histochemical characteristics, presence of metastases, presence of acid phosphatase, preliminary data revealing the presence of receptors and autonomous tumor models that are refractory to hormonal manipulation (analogous to patients with prostatic carcinoma no longer sensitive to hormonal manipulation). Additionally, the stability in tumor growth kinetics, as well as the histologic and biochemical parameters, is an important consideration in this tumor model for chemotherapeutic and hormonal studies and the tumor response to such modalities. Of note is the fact that this tumor model rarely occurs spontaneously in aging rats. This animal model has been successfully used in evaluating chemotherapeutic and hormonal treatment protocols (discussed in the Hormonal and Chemotherapeutic Considerations of the Nb Rat Model chapter). Finally, the combination of the Nb prostatic adenocarcinoma model with the transplantation of these tumors to nude mice will enable us to evaluate chemotherapeutic efficacy in the immunocompetent as well as the immunodepressed host.