In this study, we report that chronic ethanol intake at 20% of calories can enhance the initiation stage and at 15% of calories can enhance the promotion stage of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumorigenesis in female Sprague-Dawley rats. Ethanol consumption at 20% of calories by female rats from 25 to 53 days of age was associated with a significant increase in terminal end bud (TEB) structures and a significant decrease in alveolar bud structures of the normal mammary gland. In addition to changes in mammary gland morphology, ethanol consumption at 20% of calories also was associated with a significant increase in incorporation of [3H]thymidine into mammary DNA and a significant increase in the DNA-labeling index of mammary TEB. Therefore, specific ethanol intakes can enhance DMBA-induced mammary tumorigenesis. The enhancement of the initiation stage partly may be explained by alterations in the structural development of the normal rat mammary gland that increase susceptibility to chemically induced mammary tumorigenesis.