We investigated the relationship between body size, Na⁺,K⁺-ATPase molecular activity and membrane lipid composition in the kidney of five mammalian and eight avian species ranging from 30 g mice to 280 kg cattle and 13 g zebra finches to 35 kg emus respectively. Na⁺,K⁺-ATPase activity was found to be higher in the smaller species of both groups. In small mammals the higher Na⁺,K⁺-ATPase activity was primarily the result of an increase in the molecular activity (turnover rate) of individual enzymes, while in small birds the higher Na⁺,K⁺-ATPase activity was the result of an increased enzyme concentration. Phospholipids from both mammals and birds contained a relatively constant percentage of unsaturated fatty acids; however phospholipids from the smaller species were generally more polyunsaturated, and a complementary significant allometric increase in monounsaturate content was observed in the larger species. In particular, the relative content of the highly polyunsaturated docosahexaenoic acid [22:6(n-3)] displayed the greatest variation with body mass, scaling with allometric exponents of -0.21 and -0.26 in the mammals and birds respectively. This allometric variation in fatty acid composition was correlated with Na⁺,K⁺-ATPase molecular activity in mammals, while in birds molecular activity only correlated with membrane cholesterol content. These relationships are discussed with respect to the metabolic intensity of different sized animals.