1. To determine whether the alterations in amino acid metabolism after injury were a result of changes in protein synthesis, whole body tyrosine and individual tissue protein kinetics were estimated 24 h after three different forms of stress in rats. 2. In addition, injured rats were either starved or infused with 180 mg of nitrogen/day (as branched chain amino acids or L-alanine) to ascertain whether the mechanism and degree of nitrogen sparing were unique to branched chain amino acid administration or whether they could be attributed to the infusion of alpha-amino nitrogen. 3. In the more catabolic types of injury, increased nitrogen loss during starvation appeared to be due to both an increased plasma amino acid appearance and a greater percentage being oxidized. Rates of tyrosine incorporation into whole body protein were also enhanced and could be explained in part by increases in the fractional synthesis rate of hepatic non-secretory protein. 4. Both branched chain amino acid and L-alanine administration reduced endogenous tyrosine oxidation and improved nitrogen balance. However, branched chain amino acid administration significantly increased amino acid incorporation into whole body protein and fractional synthetic rates of skeletal muscle, kidney and hepatic non-secretory protein. 5. It is concluded that the catabolic response to severe injury is consistent with increased rates of plasma amino acid appearance and branched chain amino acid administration spares body protein by improving amino acid utilization for whole body protein synthesis.