Summary The estrous cycle lengths of rats fed high-fat diets ad libitum or restricted, a grain diet or a low-fat diet were determined in two long-term studies. In the first experiment, with a 40% fat diet, the rats had prolonged estrous cycles beginning at 37–40 weeks of age caused primarily by a prolongation of diestrus. During the early stages of feeding the high-fat diet did not prolong the estrous cycles. In the second experiment, the diet contained 60% fat and the prolongation of diestrus occurred at 23–27 weeks of age. In male rats, however, high fat diets from weanling to 11 weeks of age did not affect fertility or sperm production.