Our previous studies have demonstrated that there is substantial metabolic opposition to the maintenance of an altered body weight. Leptin is a protein secreted by fat cells and the circulating concentrations of leptin are directly proportional to fat mass. Leptin-deficiency is associated with severe obesity in rodents and in humans and the obesity is relieved by leptin administration. These studies examine the hypothesis that some of this metabolic opposition cto the maintenance of an altered body weight can be relieved by restoring circulating concentrations of the hormone leptin to the same range as at usual body weight in subjects who are maintaining a reduced body weight. The basic design of this study is to observe subjects at a 10% reduced body weight and then again at that reduced body weight while receiving physiological leptin supplementation.