It is generally recognized that developing animals retain sodium due to an enhanced reabsorption in distal tubule segements, even when the amount administered is in excess of their needs. This study was designed to test the relationship between this relative inability to dispose of a saline load and the functional characteristics of the kidney during postnatal maturation. In addition, we explored the role played by some of the factors known to affect natriuresis in the adult subject. Measurements of sodium excretion, glomerular filtration rate (GFR), and renal blood flow (RBF) and its intrarenal distribution were made in three age groups of puppies and in adult dogs. During expansion the GFR rose rapidly and to a similar extent at all ages, but it fell thereafter, the rate of decline being much slower in adult than in developing animals (P less than 0.001). RBF and its intrarenal distribution were not altered by volume expansion. The degree of natriuresis did not reflect either the age-related or the expansion-induced changes in GFR. Fractional and absolute sodium excretion were substantially higher in 2-wk-old puppies than in either 1- or 3-wk-old animals (P less than 0.002). These findings demonstrate that the blunted renal response of the maturing animal to saline loading is due to the persistence of an enhanced tubular reabsorption rather than to a limitation in glomerular filtration.