We report on the development of the Dunning R3327 prostatic adenocarcinoma of the Copenhagen rat as a suitable model for human prostatic cancer. Tumors produced by the subcutaneous or intraprostatic injections of viable cells had the macroscopic and microscopic characteristics of human disease. Histologically, these tumors were well differentiated adenocarcinomas with the human disease. Histologically, these tumors were well differentiated adenocarcinomas with the formation of glands and acid secretions within the acini. The intraprostatic tumor, although initially confined to the injected lobe, grew to involve the surrounding tissues and eventually metastasized to the lymph nodes and lungs. Occasional metastatic lesions were found in other organs as well. Acid phosphatase could be demonstrated by histochemical staining of frozen tumor sections and elevated levels of the enzyme were seen in the serum of rats bearing long-term subcutaneous tumors. During investigation of the tumor a fast growing line arose that grew equally as well in female as in male rats. The histology of this tumor was of an undifferentiated anaplastic tumor. Treatment by cryosurgery completely destroyed the prostatic tumor within 2 weeks. A tissue culture line derived from R3327 was capable of producing tumors in recipient rats with characteristics similar to the original Dunning tumor.