By the example of reabsorption of sodium ions in kidney, it is shown that this process, alongside with establishment of the system of osmotic concentration, plays the key role in evolution of renal function. There is shown a similarity of tendencies of changes of the total sodium ion reabsorption in children’s kidney in the process of development and in a series of representatives of various vertebrate classes. An opposite tendency is revealed during the kidney dysfunction produced by chronic renal failure. At all stages of postnatal ontogenesis in healthy children and in adults, kidney maintains the same parameters of the blood serum ion composition and osmolality. A new approach to study evolution of functions is proposed, which is based on a combination of two L.A. Orbeli’s methods—ontogenetic and clinical, by the example of study of renal function and its regulation during development of a pathological process on the background of formation of function in postnatal ontogenesis. When using such approach in children with nocturnal enuresis, significance of autacoids in regulation of urine formation is established. In the case of acute pneumonia in children, a possibility of extrapituitary production of vasopressin-like substances in lungs is shown. In norm, in postnatal ontogenesis of healthy children, in regulation of renal functions and of diuresis system, effects of efferent nerves and hormones (vasopressin) dominate, while significance of autacoids is minimal. But at a dysfunction produced by a pathological process, a change of ratio of activities of these regulatory systems is revealed. The obtained data indicate importance of the clinical and experimental methods proposed by L.A. Orbeli in analysis of evolution of renal functions and study of the nature of regulations. A combination of several methods of evolutionary physiology in one study allows revealing new mechanisms underlying regulation of function and promotes formation of concepts about regularities of evolution of functions.