The effects of dietary fat on experimental mammary cancer have typically been observed in relatively young animals. However, in human populations, breast cancer incidence and mortality are highest in postmenopausal women. To develop an animal model that stimulates the human situation more closely, female Sprague-Dawley rats were given a relatively small dose (1.5 mg) of 7,12-dimethylbenz[a]anthracene (DMBA) at 50 days of age while on a semipurified diet containing 3% sunflower-seed oil. One week later, half of the 70 rats were transferred to a diet containing 20% sunflower-seed oil. Very few mammary lesions appeared until about 35 weeks after administration of DMBA, at which time palpable mammary nodules began to appear in many of the animals on the high-fat diet. More than half of the animals in this group had developed nodules by Week 41, whereas the other half of the animals on the low-fat diet developed nodules by Week 46. Rats on the high-fat diet gradually became much more obese than those on the low-fat diet and were significantly heavier at the time they developed lesions. The incidence of nodules continued to increase in both groups and reached 100% in the group fed the high-fat diet by Week 55, with a 70% incidence of adenocarcinomas. At this time, 79% of the animals on the low-fat diet had palpable nodules without a plateau in incidence being reached. On autopsy, adenocarcinomas were found in 57% of animals on the low-fat diet.(ABSTRACT TRUNCATED AT 250 WORDS)