Renal sodium wasting has been observed in both proximal and distal renal tubular acidosis (RTA), although few studies have been reported indicating the tubular localization of such a defect. The use of clearance methodology during hypotonic saline diuresis may give an indirect estimate of proximal tubular reabsorption of sodium, sodium reabsorption at the diluting segments and proportion of sodium load reabsorbed distally. This study was carried out in 17 normal children, in 9 children with proximal RTA, associated in all but one with the Fanconi syndrome, and in 5 children with primary distal RTA. Patients with proximal RTA presented mainly an impaired reabsorption of sodium in the proximal tubule, which was in great part but not completely compensated by an absolute increase in distal sodium reabsorption. Patients with distal RTA showed normal reabsorption of sodium in the proximal tubule but they were unable to reabsorb completely the load of sodium escaping proximal reabsorption due to a defect of sodium reabsorption in the distal diluting segments. These results indicate that the classification of RTA in proximal and distal types is also valid according to the differences found in the tubular handling of water and sodium.