The effect of metabolic acidosis (MA) on amino acid and keto acid metabolism was studied in fourteen patients with chronic renal failure (CRF) under the low protein diet (0.6-0.8 g/kgBW). The comparative study of five patients with renal tubular acidosis was carried out. Each patient was investigated before [MA(+)period] and after correction with sodium bicarbonate administration lasting 10 days [MA(-)period]. The correction of MA improved nitrogen balance and elevated plasma branched-chain amino acids (BCAA), keto acids (BCKA), glutamine and alanine concentrations. No effect was however, observed in change of plasma insulin and glucagon. Oral administration of the keto-analogues of BCKA [0.1 g/kgBW of alpha-ketoisovalerates (KIV) and alpha-keto-isocaproic acid (KIC)] is made for the purpose of investigating the change in the metabolic conversion rate to amino acids. As a result, MA (+) suppressed an increase in plasma KIV and KIC concentrations. Moreover, an increase in plasma valine and leucine concentrations were suppressed by MA (+). These results suggested that MA stimulates BCKA oxidation and suppresses the protein sparing effect of leucine and KIC, and accelerates the catabolism in CRF under the low protein diet. The correction of MA is ineffective in severe renal failure (serum creatinine above 10.0 mg/dl), because the other uremic factors appear to be affecting protein and amino acid metabolism. Therefore, it might be concluded that MA should be corrected at an earlier stage of CRF.