The ACI rat is a unique model of human breast cancer in that mammary cancers are induced by estrogen without carcinogens, irradiation, xenografts or transgenic manipulations. We sought to characterize mammary cancers in a congenic variant of the ACI rat, the ACI.COP-Ept2. All rats with estradiol implants developed mammary cancers in 5-7 months. Rats bearing estradiol-induced mammary cancers were treated with tamoxifen for three weeks. Tamoxifen reduced tumor mass, measured by magnetic resonance imaging, by 89%. Tumors expressed estrogen receptors (ER), progesterone receptor (PR), and Erbb2. ERalpha and PR were overexpressed in tumor compared to adjacent non-tumor mammary gland. Thus, this model is highly relevant to hormone responsive human breast cancers.