In order to follow early metabolic adaptations in adipose tissue, which may lead to a decrease in fat cell size and body fat obtained by physical training, two sets of experiments were performed. Obese subjects and a control group exercised on a bicycle at two-thirds of maximal working capacity for one hour. Twenty-four hours thereafter, either on ad libitum diet, or on an isocaloric diet supplemented with calories corresponding to the expanded calories during the work load, an adipose tissue biopsy was taken and fat cell metabolism studied. In obese subjects on and ad libitum or on an isocaloric diet the lipolytic process was increased after the exercise but no significant effects were found on the rates of glucose metabolism. Furthermore, a relationship between cell surface and metabolism was found before, but not after, the work in all groups. No changes were seen in lipoprotein lipase activity. Obese subjects on a controlled diet showed a somewhat higher insulin and catecholamine responsiveness of adipocytes than was the case in the obese subjects on an ad libitum diet. This may well be due to the differences in carbohydrate intake, a factor of importance for hormonal responsiveness of fat cells. The increased basal lipolysis after exercise may be caused by the release of lipolytic hormones, and may well be the first sign of an adaption of the organism to diminish the fat stores as seen in physically trained subjects.