The induction of murine mammary carcinogenesis by 1-methyl-1-nitrosourea was blocked by the feeding of a purified diet formulation supplemented with 25 p.p.m. vanadium as vanadyl(IV) sulfate during the post initiation stages of the neoplastic process. Treatment with vanadyl(IV) sulfate reduced both cancer incidence and the average number of cancers per rat and prolonged the median cancer-free time without inhibiting the overall growth of the animals. Vanadyl(IV) sulfate appears to be an effective non-toxic agent for the chemoprevention of experimental breast cancer in the rat.