Increased fat mass, abdominal adiposity, and insulin resistance are typical findings in aging mammals and are frequently associated with leptin resistance and increased plasma leptin levels. To examine whether leptin’s failure in aging is due to aging per se or to changes in body fat mass or distribution, we studied aging rats that underwent calorie restriction throughout their lives, maintaining their youthful body fat pattern and metabolic profile. Leptin’s action was assessed by measuring its ability to regulate food intake, fat mass and its distribution, peripheral and hepatic insulin action, and its own gene expression in fat. Our results show that leptin’s action is markedly diminished in aging rats, independently of their body fat pattern. Leptin’s failure in this model suggests its causative role in the metabolic decline seen with aging.