Oxygen consumption of tissue slices of liver, kidney, and brain were measured at 37°C to assess in vitro metabolism of tissues of Amphibolurus nuchalis and Mus musculus (a reptile and mammal with same weight and body temperature). 'Sodium transport' metabolism was also assessed (with use of ouabain) in these tissues. The mammal had an in vitro tissue metabolism that was two to five times that of the reptile and its sodium transport metabolism was four- to ninefold greater. Growth of reptile young and mammal young at the same body temperature was measured and the mammal showed a growth rate that was an order of magnitude greater than the reptile. Thyroid activity was assessed in the reptile and mammal by measurement of thyroidal uptake and release of 125I and serum thyroxine concentration. All parameters indicated a greater thyroid activity in the mammal. These findings are discussed in relation to the action of the thyroid hormones, the paleoecology of mammals and reptiles and the evolution of endothermy.