BALB/cCrgl and C57BL/Ki mice were treated 4 weeks after pituitary isograft with urethan, 3-methylcholanthrene, or 7,12 -dimethylbenz [a] anthracene (DMBA). Among BALB/c mice treated with 6 mg DMBA, mammary hyperplastic alveolar nodules (HAN) were rare, occurring in only 30% (an average of only 1 per mouse); mammary adenocarcinomas and adenoacanthomas were frequent (68%). In C57BL mice similarly treated, the mammary tumor incidence was 32%, and HAN were rare. Both strains showed atypical intraductal hyperplasias, particularly C57BL mice exposed to urethan. A third type of mammary dysplasia seen in carcinogen-treated BALB/c mice was characterized by squamous metaplasia of the mammary alveoli. These lesions had mononuclear cell infiltration, varying degrees of fibrosis, and significant areas of pyknotic nuclei.