Studies of the vascularization of autochthonous rodent mammary tumors are limited in number, and the majority have used Factor VIII staining for blood vessel detection. Moreover, little effort has been directed at measuring the vascularization of tissue immediately adjacent to a tumor despite its central importance in the process of angiogenesis. Thirty-six chemically-induced mammary carcinomas and tissue immediately adjacent to these carcinomas were used to develop a census counting method for quantitative assessment of intra- and extra-tumor vascularization. Blood vessels were identified using antiserum directed against either CD31 or Factor VIII. Techniques used to create digitized images of all tumors and the semi-automated methods for circumscribing the extra-tumoral region are described. For Factor VIII, CD31 allowed greater discrimination of blood vessels with areas <25 microm(2) and demonstrated crisp staining of blood vessels, with minimal background and excellent preservation of tissue architecture. Census counting data support the use of CD31 for quantifying both intra- and extra-tumoral vascularization. This method provides a basis for standardizing the approach to evaluation of experimentally induced premalignant and malignant mammary lesions in rodent model systems used to investigate potential anti-angiogenic cancer preventive agents.