• To evaluate the effects of protein intake on renal tubular functions, seven metabolic studies were performed on five adolescents with mild, chronic renal insufficiency. After equilibration, patients were randomly assigned to two periods of four days each on either a high-protein diet (HPD) or a low-protein diet (LPD), with energy intakes remaining the same during both periods. High protein intake resulted in significantly higher mean (±SD) daily creatinine excretion and clearance rates. The HPD was associated with significantly increased excretion rates of potassium, phosphate, sulfate, and calcium. Net acid excretion with the HPD was nearly threefold higher than that observed with the LPD. These differences were unassociated with urinary pH, which remained similar with both diets. We conclude that different protein intakes affect urinary excretion of creatinine, potassium, phosphate, sulfate, calcium, and net acid.