Mammary tumor induction was examined in female Fischer rats fed a low-corn oil, a high-corn oil, a high-lard, a high-beef tallow, or a high-coconut oil diet since weaning. The diets were prepared by adding the experimental fat to a basal diet containing sufficient essential fatty acids for growth. These diets differed only in the concentration or type of dietary fat. The rats were given a single i.v. dose (50 mg/kg body weight) of N-nitrosomethylurea at 50 days of age. Mammary tumor incidences 28 weeks after N-nitrosomethylurea treatment in rats on low-corn oil, high-corn oil, high-lard, high-beef tallow, and high-coconut oil diets were 33, 85, 65, 50, and 43%, respectively. The data show that an increase in fat intake enhances mammary carcinogenesis, but the magnitude of the increase depends on the type of fat. Further analyses showed that the total oleic and linoleic acid intake in the five groups of rats correlated positively (r = 0.95) with mammary tumor incidence, whereas the composition of the mammary tissue neutral lipids and phospholipids did not. Our data suggest that the total oleate and linoleate intake in the high-fat diet is the major factor influencing the incidence of tumors by N-nitrosomethylurea.