LEATHERBACKS (Dermochelys coriacea) are among the largest living reptiles (>900 kg)1, 2 and range from the tropics to north of the Arctic Circle3, 4. They maintain elevated body temperatures (25.5 °C) in cold seawater (7.5 °C)5, 6 and heat up on land7. Metabolic and thermoregulatory mechanisms of leatherbacks have important implications for considerations of size and function in animal biology8-10 and for speculation on the endothermic capacities of dinosaurs11-18. Here we report that metabolic rates of adults at rest and while nesting are intermediate to those predicted by allometric relationships for reptiles and mammals. Mathematical modelling indicates that leatherbacks can use large body size, peripheral tissues as insulation, and circulatory changes, to maintain warm temperatures in the North Atlantic and to avoid overheating in the tropics. This 'gigantothermy' probably allowed large dinosaurs to live in varied habitats, including Cretaceous polar regions.