The relationship between dietary fat and mammary carcinogenesis was examined by feeding a high-fat or a low-fat purified diet, differing only in fat and carbohydrate content, to Fischer, Long-Evans, and Sprague-Dawley rats. An additional group of rats from each strain was fed a nonpurified laboratory diet for comparison. The rats were given an i.v. dose of N-nitrosomethylurea (50 mg/kg body wei...