[Objective] The present work is to establish rat spontaneous mammary adenocarcinoma tumor strain to produce the type C oncovirus and investigate the pathological characteristics of rat spontaneous mammary adenocarcinomas caused by type C oncovirus. Methods: The tumor strain of rat mammary adenocarcinomas was established by alternate descent generations of carcinoma struma and cell suspension. For pathological observation, rat mammary primary adenocarcinoma and implant adenocarcinoma were respectively fixed in 10% formaline, embedded in paraffins, routinely stained with HE and Ag. For electron microscope observation, rat mammary primary carcinoma and implant carcinoma were double fixed in 2.5% glutaraldehyde and 1% osmic acid respectively, embedded in Epon 812, ultrathin sectioned, double stained with uranium acetate and lead citrate, and observed under transmission electron microscope (TEM). [Results] The volume of rat mammary primary carcinoma cells is quite small, only 10 (m in diameter. The typical tubular structures are found to be in such shapes as of globe, pappila, tortuousribbon and whorl. The carcinoma cells are separated into irregular nest structure by the reticular fibres after Ag staining. The above-mentioned shows that, as a type of rat mammary epithelial anaplastic adenocarcinomas, rat mammary carcinoma is originated from epithelial tissues. The implant carcinoma cells are round, cubic, and polygonal in shape, and carcinoma gigantic cells, pale cells and the like appeared, which are arranged in such structures as of nest, layer, trabecula, pappila, glomerate, adenocarcinoma tubule, and space, with heteomorphosis notable. The carcinoma cells are irregular in size, with large nuclei and increased nucleolus. Besides, there are bundles of tension protofibril in cytoplasm and seldom seen secretory granules. Among the cells there are primary gland cavities and the desmosome. The type C oncovirus particles are discovered among the carcinoma cells and in their cytoplasm, which are 100nm in diameter, in the shape of globe and with external envelopes endomembranes and central cores. [Conclusion] This is the first time that rat spontaneous mammary adenocarcinomas have been established to generate type C oncovirus tumor strain. It makes up a type of rat mammary epithelial anaplastic adenocarcinomas. And its pathological morphology is of polymorphosm.