1. 1. The metabolic capacities of reptilian and mammalian hearts have been investigated using two methods: measurement of mitochondrial enzyme activity (cytochrome oxidase) and measurement of both mitochondrial volume density and membrane surface area. 2. 2. The heart tissues from the reptiles and mammals showed 2-fold "weight specific" and 3-fold total organ metabolic capacity differences. 3. 3. Heart mitochondria from reptiles and mammals showed 2-fold differences in the activity of their enzymes per mg of mitochondrial protein yet showed very similar mitochondrial surface areas per cm3 of mitochondria. 4. 4. Heart mitochondria differ from liver mitochondria which have the same enzyme activities per mg of protein and the same mitochondrial surface area per cm3 of mitochondria in both the reptiles and mammals. 5. 5. A wide variety of reptiles and mammals both showed relationships between total heart metabolic capacity and body weight. 6. 6. Mammals have larger hearts than similar sized reptiles and their hearts have a greater proportion of cellular volume occupied by mitochondria. © 1983.