AbstractThe effect of low‐fat and high‐fat diets on the induction of mammary carcinomas by 1‐methyl‐1‐nitrosourea (MNU) was studied in female Sprague‐Dawley rats. All rats were given MNU (25 mg/kg body wt) at 50 days of age. For the first 17 weeks after carcinogen administration, they were fed a purified diet containing either 5 or 20% fat incorporated into agar gel. Food intake was restricted, so that the amounts fed provided the same amount of net utilizable energy each day for both groups, regardless of the fat content of the diets. From 17 to 32 weeks, the diets were fed ad libitum. During the restricted feeding period, there was no significant difference in tumor incidence or in the number Of tumors detected between the groups. During the weeks in which animals were fed ad libitum, significantly more tumors appeared in the high‐fat group than in the low‐fat group. The data provide support for the hypothesis that consumption of a high‐fat diet can lead to an enhancement of mammary carcinogenesis. It appears, however, that diets must be consumed ad libitum for the stimulatory effect on tumor occurrence to be exhibited.