In this investigation, the effects on proton leak of leptin administration to ob/ob mice was measured for liver mitochondria. We and others have shown that proton leak is approximately 3 times greater in liver mitochondria from ob/ob mice compared to lean controls at any given membrane potential. The results are consistent with obese mammals having higher lean mass-specific metabolic rates compared to lean controls. The increase in proton leak rate at any given membrane potential cannot be explained by the presence of free fatty acids associated with mitochondria isolated from the obese animals. The difference in proton leak must therefore represent a real difference in inner membrane permeability. Acute leptin (OB protein) administration restores the liver mitochondrial proton leak rate of ob/ob mice to that of lean controls. There was no effect on proton leak rate of liver mitochondria from sham-treated ob/ob mice. These novel results indicate a role for leptin, either directly or indirectly, in regulating the efficiency of oxidative phosphorylation.