Energy intake, utilization, and partitioning were determined in male Wistar rats from 25 to 180 d of age. Serum free triiodothyronine, leptin, and free fatty acid concentrations were also measured. Energy balance measurements allowed us to identify a period from 25 to 90 d, characterized by a rapid body growth rate and another from 90 to 180 d, during which body growth rate slowed. From 25 to 180 d, we found decreases in daily energy intake and expenditure, which were faster before 90 d. The first period was characterized by storage of lipid and protein. In the second period, protein deposition approached zero and the excess of ingested energy was entirely stored as fat, so that age-associated obesity began to develop. The inability of rats to maintain a stable body weight after the cessation of growth of lean body mass is not due to decreased resting metabolism but rather to a partial leptin resistance.