The goal of nutritional support in sepsis is, as in other conditions, to prevent the use of endogenous protein as an energy substrate and, ideally, to promote the synthesis of proteins specifically required in responding to the particular insult or stress at hand. This entails provision of an utilizable fuel, in sufficient quantity, that does not inhibit the use of endogenous nonprotein sources; preservation of the existing protein mass by minimizing skeletal muscle and visceral proteolysis; provision of amino acids in sufficient quantity and in the appropriate proportions such that protein synthesis is optimized. Specifically, this includes the synthesis of those proteins required to maintain hyperdynamic function of the essential organs as well as the hepatic and leukocytic synthesis of proteins required in immunologic defense. This study has assessed one aspect of this goal during the administration of nutrient solutions differing primarily in branched chain amino acid content. We conclude that leucine is fundamental among the branched chain amino acids for reducing skeletal muscle proteolysis. Solutions designed for sepsis or stress should, therefore, contain adequate amounts of this amino acid.