Angiotensin II receptors of the AT1 subtype were very highly expressed in medroxyprogesterone-induced ductal adenocarcinomas of the mammary gland in BALB/c mice. AT1 receptors are associated only to neoplastic epithelial cells. Lobular adenocarcinomas expressed very few AT1 receptors and expressed AT2 receptors only in areas corresponding to peritumoral connective tissue. Binding to angiotensin converting enzyme was present in all adenocarcinomas studied and was higher in ductal than in lobular adenocarcinomas. Normal mammary gland did not express either angiotensin II receptors or angiotensin converting enzyme. The present results are the first demonstration of angiotensin receptor subtypes and converting enzyme in mammary adenocarcinomas differentially expressed in tumors of ductal and lobular origin. Localization of receptor subtypes could be useful to study the differentiation of mammary cells during experimental mammary carcinogenesis in mice.