The rat mammary gland epithelium is composed of three cell types: dark cells (DCs), intermediate cells (ICs), and a layer of myoepithelial cells (MCs), which are evenly distributed along the mammary gland tree in rather constant proportions. The present study was carried out for a determination of the effect of the carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) on the distribution and proliferative activity of these cell populations. The proportion and the DNA labeling index (DNA-LI) of each cell type were determined in terminal end buds (TEBs), terminal ducts (TDs), alveolar buds (ABs), and alveoli of the normal Sprague-Dawley rat mammary gland and in intraductal proliferations (IDPs) and carcinomas removed at selected intervals after DMBA administration. DMBA-induced changes in cell distribution were limited to TEBs and TDs, whereas ABs and alveoli were unaffected. The alterations consisted in an increment in ICs from 11% in TEBs and TDs to 90% in tumors and a decrease in DCs from 77% in TEBs and TDs to 7% in tumors. MCs were relatively unaffected. The DNA-LI of DCs, which in the normal gland TEB was 14%, was depressed by DMBA to 6%, whereas the DNA-LI of ICs remained unchanged from the basal level of 40% during the process of carcinogenesis. The progressive increment in number of ICs with a steady DNA-LI suggested that the IC is the target cell of the carcinogen and the cell of origin of mammary carcinomas.