The renal clearance of 21 amino acids was investigated in 5 patients with chronic renal failure (inulin clearance ≈ 5–10ml/min) during the 6-week administration of a low-protein diet. The results were compared with values obtained in 6 volunteers. The renal clearance of all investigated amino acids (with the exception of Asp, His, Tau) did not differ significantly from healthy controls. The excretion fraction (amino acid clearance /inulin clearance 100) was in all investigated amino acids significantly higher than incontrols with the exception of Tau. The excretion fractions of Asp, Ser, Cys, Tyr, His, Thr, Tau and Gly were, on average, above 20%.The findings suggest that the fractional reabsorption of amino acids in residual nephrons is decreased.