The level of Na reabsorption, the succinate dehydrogenase and Na, K ATPase activity, the content of phospholipids, and their fatty acid composition was studied in the kidneys of representatives of different classes of vertebrates. The level of Na reabsorption in the kidneys of the warm blooded animals studied (chicken, rat), was considerably higher than in the kidneys of cold blooded animals (lamprey, red salmon, perch, frog, tortoise). A study of the activity of the enzymes at 25°C showed that the succinate dehydrogenase activity, but not the Na, K ATPase activity increases in the series of animals investigated from lower to higher. The content of phospholipids in the kidneys of the animals investigated, in μg P/g tissue, varied from 750 to 1220 μg/g. The largest amount of phospholipids was found in the cortex of the rat kidney. A special feature of the fatty acid composition of the kidney phospholipids of cold blooded as compared with the warm blooded vertebrates was their higher content of monoenoic acids.