O2 consumption was measured at 37°C in tissue slices of liver, kidney, and brain from Amphibolurus vitticeps and Rattus norvegicus (a reptile and mammal with same weight and body temperature) both the in the presence and absence of ouabain. O2 consumption of the mammalian tissues was two to four times that of the reptilian tissues and the mammalian tissues used three to six times the energy for Na+-K+ transport than the reptilian tissues. Passive permeability to 42K was measured at 37° C in liver and kidney slices, and passive permeability to 22Na+ was measured at 37°C in isolated and cultured liver cells from each species. The mammalian cell membrane was severalfold 'leakier' to both these ions than was the reptilian cell membrane, and thus the membrane pumps must use more energy to maintain the transmembrane ion gradients. It is postulated that this is a general difference between the cells of ectotherms and endotherms and thus partly explains the much higher levels of metabolism found in endothermic mammals.