Summary.1The fact that other investigators have found a proportionality between kidney weight and kidney function, and between kidney weight and surface area in animals is referred to.2Investigations into the kidney function (determined by estimating the amount of filtrate) in 90 normal children from 9 months to 17 years of age, are discussed.3It is shown that the amount of filtrate is roughly proportional to the surface area. The minimum is 60 cc. per sq.m. per minute. There are wider variations in the values for the filtrate compared with the body weight, the minium being 2,08 cc. per kg. per minute.4It is not certain that the amount of filtrate per sq.m. diminishes with age, although a downward tendency seems to be indicated; the amount filtrate per kg. shows a fall of 25—30 %>from the age‐group 2—5 years to the age‐group 11—14 years.5On the basis of earlier investigations, it is shown, that, in man, the kidney weight is proportional to the body surface; that the kidney weight per unit surface is almost constant at different ages; and that the kidney weight per kg. diminishes 25—30 % with the age in the age‐groups mentioned. It is pointed out that one can therefore also assume that the kidney function is proportional to the kidney weight in man.6Parallels between kidney function, metabolism and kidney weight at different ages are drawn, and the facts presented make it probable that kidney weight and kidney function, as was assumed by MACKAY, are determined by the standard metabolism.7Concentration tests in 10 children of 4—14 years of age are discussed. The concentrating power of children is the same as that of adults, and is independent of the size of the child.8The clinical importance of determinations of the quantity of filtrate in children suffering from renal diseases is discussed. An amount of filtrate of under 60 cc. per minute per sq.m. must be regarded