2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a heterocyclic amine derived from cooked meat. Mammary gland cancer can be induced in female Sprague-Dawley rats by administration of several oral doses of PhIP. The mechanism of mammary gland carcinogenesis by PhIP in this rat model is not fully understood but appears to involve several factors. One factor is the formation of PhIP-DNA adducts in the mammary gland after metabolic activation of PhIP. Possible target cell populations include the epithelial cells of the mammary gland terminal end buds (TEBs), putative sites of origin of carcinomas. Another factor involved in the mammary carcinogenicity of PhIP may be an increased proliferation in epithelial cells of the TEBs which occurs after a carcinogenic dose of PhIP is administered. This proliferation would be likely to enhance the fixation of mutations from PhIP-DNA adducts in target cells and facilitate the initiation of carcinogenesis. PhIP exposure also transiently inhibits the development of the mammary gland by retarding the differentiation of TEBs to alveolar buds and lobules. As a consequence, more TEBs are available for neoplastic transformation. Recent studies in rats have also shown that PhIP increases the levels of serum prolactin, a well-recognized promoter of mammary gland cancer, which may further explain the targeting of PhIP to the mammary gland. The results to date indicate that PhIP has multiple effects on the mammary gland and hormone status in rats that could potentially play a role in its ability to induce mammary gland cancer.