Body composition was measured and compared in Amphibolurus vitticeps and Rattus norvegicus (a reptile and a mammal with the same weight and body temperature). Homogenates were prepared from liver, kidney, brain, heart, lung, and skeletal (gastrocnemius) muscle, and mitochondria were isolated. Cytochrome oxidase activities of both tissue homogenates and isolated mitochondria were measured (at 37°C) as was protein content. Phospholipids were extracted from liver and kidney, and the fatty acid composition was determined. The brain, liver, kidney, heart, and skeletal muscle were significantly larger in the mammal, whereas the skin, reproductive organs, lung, and digestive tract showed no significant difference in size. All mammalian tissues examined contained ~ 50% more protein and phospholipid than the respective reptilian tissue. Although the mammalian phospholipids contained significantly less total unsaturated fatty acids, these unsaturated fatty acids were significantly more polyunsaturated than in the reptilian tissues. Tissue cytochrome oxidase activity was significantly greater in mammals when expressed on a wet weight basis but not when expressed on a tissue protein basis. Mitochondrial cytochrome oxidase activity (on a protein basis) was the same in both species in liver, kidney, and brain, but in heart, lung, and skeletal muscle mammalian mitochondria were twice as active as reptilian mitochondria. The implications of these differences in tissue composition were discussed to the evolution of mammalian endothermy.