Leptin-resistant rats, when given a high-fat (HF) diet, have a delayed normalization of caloric intake and greater weight gain than those on a chow diet. Because aged, obese rats are leptin resistant, these data predict that they will also have a delayed normalization of caloric intake and exacerbated weight gain when provided a HF diet. To investigate this hypothesis, along with the consequences of a HF diet on voluntary wheel running, we compared various ages of rats on a HF or chow diet. HF-fed young rats spontaneously divided into diet-induced obese and diet-resistant rats. However, all aged rats were susceptible to the weight-gaining effects of HF feeding. Rate of initial weight gain was proportional to age, and peak caloric intake on the HF diet and the days required to normalize caloric intake to basal levels increased with age. Responsiveness to peripheral leptin before HF feeding revealed a dose-response decrease in food intake and body weight in the young but no responses in the aged to even the highest dose, 0.5 mg/day. In addition, both age and HF feeding decreased the tendency for wheel running, suggesting the propensity for inactivity with age and HF feeding may contribute to age-related obesity and accelerate the rate of diet-induced obesity. These results demonstrate that aged rats are more susceptible to the detrimental effects of a HF diet.