Environmental exposure to nitrosamines is associated with the development of cancer in a variety of target organs. One such carcinogen, N-methyl-N-nitrosurea (MNU), has long been used to induce mammary tumors in rats, which provide a useful model to study mammary carcinogenesis. However, some poorly clarified issues remain, such as the lack of a clear description of morphological patterns of tumors and the distribution and role of estrogen receptors (ERs) during tumor progression, as tumors overexpressing ERs show a paradoxical tendency to recur after ovariectomy. Mammary carcinomas were induced in Sprague-Dawley rats using MNU. The tumors were studied histologically and distribution of smooth muscle actin and ERs was studied immunohistochemically. All tumors presented both an epithelial and a myoepithelial component, demonstrated by immunohistochemical detection of smooth muscle actin. Tumors showed distinct histological patterns: well-differentiated papillary and adenoid areas and poorly differentiated solid and spindle-cell foci. Overexpression of ERs (>75% of labelled cells vs. 0-75% in control tissue) occurred in papillary and adenoid areas but not in solid and spindle-cell foci. Poorly differentiated tumor foci are likely to represent a more advanced, estrogen-independent phase in cancer progression and constitute the basis for tumor recurrence after ovariectomy.