The precancerous changes of mammary glands in 7,12-dimethylbenz(alpha)anthracene (DMBA) induced carcinogenesis in Wistar rats were examined. Carcinogen was inserted into the left fifth mammary gland of the anesthetised rats. After 35 days all the animals were sacrificed and mammary glands were extirpated. Macroscopic examination of mammary glands was performed and the tissue was processed for a pathohistological analysis. H&E, VanGieson's and Toluidine-blue methods were applied, as well as ABC immunohistochemical method with anti-cytokeratin antibodies.The identified precancerous changes resembled to aberrations of fibrocystic disease in women. The fibrosclerosis, lobular hyperplasia, cystic ductal dilatation and apocrine metaplasia of ductal epithelium were found. Micropapillomatoid hyperplasia was another frequent finding, but the presence of the real papilloma was not registered. The keratocysts with the squamous epithelial metaplasia were present in three of the animals. Dysplastic changes were found in the skin, above the treated glands. The difference in expression of cytokeratins, between normal and preneoplastic epithelium, makes cytokeratin useful for verification of early precancerous lesions. The epithelial ductus and ductulus cells of the mammary glands of animals belonging to the control group showed neither CK 19 nor CK 14 expression.