The fat content of young, growing rats can be kept extremely low by regularly performed endurance exercise. The reduction in fat, as reflected by studies performed on epididymal fat pads of rats, is due to a decrease in both the size and number of adipose cells. The finding that exercise can modify adipose tissue cellularity motivated us to continue work in this area since a permanent reduction in adipose tissue cellularity in early life can have interesting implications with respect to the development of severe obesity in adulthood. Our plan is to elucidate the role of exercise in curbing the potential for obesity. In the course of completing the work outlined above, preliminary studies have been designed to obtain leads regarding the basic mechanisms by which exercise modifies adipose tissue cellularity. This work will lead us to study the interrelationships between exercise and nutrition with respect to weight gain, adipose tissue cellularity, and body composition.