Female obese (ob/ob) and lean mice were weaned at 21 days of age, placed in wire-mesh cages maintained at 25 to 30 degrees, and fed a high-fat or a high-carbohydrate diet for 21 days. The body energy balance procedure was utilized to determine the maintenance energy requirements, and the efficiency of dietary energy utilization, above maintenance, in these mice. Heat production of each mouse was measured weekly in a gradient-layer calorimeter. Regressions of changes in body energy per kg3/4 on metabolizable energy intake per kg3/4 indicated that the maintenance energy requirement averaged 72 kcal/kg3/4/day for obese mice and 124 kcal/kg3/4/day for lean mice. Diet composition did not influence the maintenance energy requirements, but utilization of energy, above maintenance, in obese mice fed the high-fat diet was 41% more efficient than observed in obese mice fed the high-carbohydrate diet and 38 to 71% more efficient than observed in lean mice. Heat production, per unit body weight was lower in obese mice than in lean mice. The lowest heat production was observed in obese mice fed the high-fat diet. The 40% lower maintenance energy requirement of the obese mice is a major factor contributing to the high efficiency of energy retention in these mice. Consumption of a high-fat diet further improved the ability of the obese mice to retain dietary energy.