Age-related changes in mitochondria, including decreased respiratory control ratios and altered mitochondrial inner membrane lipid composition, led us to study oxidative phosphorylation in hepatocytes from old (30 mo) and young (3 mo) male C57BL/J mice. Top-down metabolic control analysis and its extension, elasticity analysis, were used to identify changes in the control and regulation of the three blocks of reactions constituting the oxidative phosphorylation system: substrate oxidation, mitochondrial proton leak, and the ATP turnover reactions. Resting oxygen consumption of cells from old mice was 15% lower (P < 0.05) than in young cells. This is explained entirely by a decrease in oxygen consumption supporting ATP turnover reactions. At all values of mitochondrial membrane potential assessed, the proportion of total oxygen consumption used to balance the leak was greater in the old cells than in the young cells. Metabolic control coefficients indicate a shift in control over respiration and phosphorylation away from substrate oxidation toward increased control by leak and by ATP turnover reactions. Control of the actual number of ATP molecules synthesized by mitochondria for each oxygen atom consumed by the ATP turnover and leak reactions was greater in old than in young cells, showing that efficiency in older cells is more sensitive to changes in these two blocks of reactions than in young cells.