Spontaneous adenocarcinomas of the ventral prostate were present in 7 of 41 aged (34- to 37-month-old), virgin, untreated male A X C rats. The only consistent gross evidence of possible neoplastic involvement was intraprostate hemorrhage. The principally intraglandular neoplasms were composed of markedly anaplastic epithelial cells which retained a moderate propensity to form glandular patterns. The nuclei of the neoplastic cells were pleomorphic, vesiculated, enlarged, and hyperchromatic. Mitotic figures were frequent. Interglandular connective tissue was invaded in one rat; however, metastases were not demonstrated