Intraperitoneal administration of high doses (45, 90, and 180 mg/kg of ENU (N-ethyl-N-nitrosourea) to 30-day-old female Sprague-Dawley (CD) rats resulted in a high incidence (greater than 100%) of mammary tumors (MT), of which 95% were malignant. The mean induction time was 92 days for the high dose group. There was a definite dose-effect relationship. As the dose of ENU decreased the number of rats affected and number of tumors per rat decreased, but the survival time and mean induction time increased. At the time of death 7.5% of mammary tumors had metastasized to the lung. A variety of histological MT types were produced simulating breast tumors in women. In addition to MT, a number of rats developed primary tumors in other organs (brain, ovary, kidney, and hemic-lymphatic system). The high MT incidence, high rate of malignancy, short latent period, and the simulation of human breast tumor types suggest this experimental design to be a valuable animal model to study initiation, hormone dependency, and pathogenesis of mammary neoplasia.