The characteristic amino acid pattern observed in chronic liver failure with high aromatic and low branched chain amino acid levels is considered to be consequent to increased muscle protein catabolism. The main catabolic stimulus has been attributed to hyperglucagonemia and to a reduced insulin/glucagon molar ratio. Intravenous administration of a solution containing branched chain amino acids and glucose to patients with chronic liver cirrhosis rapidly normalizes the plasma amino acid pattern. This effect may result from either a change in the insulin/glucagon ratio, induced by glucose, or from the anticatabolic influence of branched chain amino acids on muscle protein turnover. To discriminate between these two possibilities, a crossover study was carried out to determine the effect of a 24-hour infusion of either glucose alone, or glucose plus branched chain amino acids, in seven patients with chronic liver failure. Blood glucose, insulin, glucagon, free fatty acids, and amino acid levels were determined. Branched chain amino acids were much more effective than glucose (p less than 0.01) in decreasing the levels of aromatic amino acids. Conversely, the insulin, glucagon, and free fatty acid levels with glucose alone were not altered with the addition of branched chain amino acids. These findings suggest an anticatabolic effect of branched chain amino acids on muscle protein turnover and suggest that factors other than insulin and glucagon may be responsible for the characteristic plasma amino acid pattern present in chronic liver failure.