Food intake, body weight, body composition, resting metabolic rate (RMR) and the thermic effect of food (TEF) were measured in young rats, some of which were fed a high energy (HE) diet and some of which were forced to swim daily. In general, high energy feeding as compared to chow feeding, resulted in higher food intake, higher body weight, higher body fat, and a slightly lower TEF. In many cases, however, the specific effects varied with the age and sex of the animals. Animals forced to swim weighed less; were leaner; and had higher RMR and TEF than sedentary animals. The effects of exercise on energy balance were greatest in males, while the effects of the high energy diet on energy balance were greatest in females. All HE-fed rats were switched to lab chow at 104 days of age. Body weights of sedentary HE-fed rats returned to control levels but those of exercised HE-fed rats did not. Both HE-fed groups remained fatter than chow-fed controls, even two months after the diet switch. © 1984.