The ability to induce formation of new vessels was tested in fragments of rat mammary tissue transplanted onto the rabbit iris and observed through the transparent cornea. Virgin, pregnant, and lactating glands showed an angiogenic capacity in about 5% of implants. In contrast mammary carcinomas induced angiogenesis in 75 to 100% of implants. Fragments of mammary gland previously treated with 7,12-dimethylbenz[α]anthracene or N-nitrosomethylurea but without histological evidence of neoplastic transformation showed an angiogenic response in about 5% of implants. The same low angiogenic response was detected in primary hyperplastic alveolar nodules. However, angiogenesis was observed 2 to 3 times more frequently in implants from hyperplastic outgrowths that acquired continuous transplantability and showed a high degree of neoplastic transformation. These data on the rat mammary gland confirm previous findings on mouse mammary gland, indicating that: (a) neoplastic epithelium has a higher angiogenic capacity than does normal epithelium; and (b) hyperplastic epithelium at high risk of undergoing neoplastic transformation induces angiogenesis more frequently than does hyperplastic epithelium with low tumor potential.