Comparative studies contribute significantly to determining principles of renal physiology. Examples of comparative relationships important for past, current, or future studies are discussed. Structural simplicity of the avian glomerulus may permit detailed analysis of filtration pressure profiles. Alterations in nephron function, redistribution of filtration among nephron populations, and control by antidiuretic hormone and other factors are readily demonstrable in nonmammalian vertebrates. Studies of the distal tubule-glomerular feedback mechanism may be particularly revealing in nephrons of nonmammalian vertebrates, many of which lack a macula densa. The influence of volume flow rate in collecting ducts on the concentrating mechanism can be analyzed readily in avian kidneys in which glomerular intermittency influences such flow. The structure of Henle's loops may be related to the importance of sodium, chloride, and urea in avian and mammalian concentrating mechanisms. Perfusion of isolated reptilian renal tubules to study urate transport has also raised questions about fluid absorption and may permit more detailed analysis of some other transport systems than perfusion of mammalian tubules. Simplicity of some nonmammalian systems and exaggerated functions in these systems should continue to make them useful for revealing basic principles of renal function.