N-nitrosomethylurea (NMU) given intravenously to rats at age 50 days induced mammary carcinomas in 89% of BUF/N, 73% of Sprague-Dawley, and 89% of F344 females. Latent periods were, respectively, 77, 86, and 94 days. Mortality was negligible. Biologic properties of NMU-induced tumors were tested in the BUF/N inbred strain. Before treatment, it reduced the number of tumors per rat but not the incidence; and after the tumor was established, castration arrested tumor growth or caused a temporary regression of the tumor. Metastases to bone marrow and spleen were constant, but they were rare to the liver and lungs. After the primary tumor was removed, metastases continued to grow but at a slower rate than the growth of the primary tumor. Almost all tumors were transplantable intraperitoneally and/or subcutaneously in the inguinal area of intact as well as ovariectomized and adrenalectomized rats. Transplanted tumors were able to metastasize as were primary tumors. Doubling times of NMU-induced primary and transplanted carcinomas were similar to 7 days. Cachexia ensued at the 5th week from the onset of the first tumor. When the tumor was larger than 15 g, hypercalcemia was usually observed. The treatment described appears to be the simplest method for inducing in rats a most nearly complete model for human mammary carcinomas.