Whether exercise, when used as an adjunct to parenteral nutrition, has any influence on postoperative muscle catabolism, amino acid metabolism and N balance was investigated. Male Sprague-Dawley rats [20] were divided into 2 groups of exercised and nonexercised animals. All animals underwent laparotomy and jugular vein cannulation, were placed in metabolic cages and were infused with a dextrose-protein solution at 15 kcal and 0.56 g amino acids/100 g body wt per 24 h for a total of 96 h. The exercised animals were run on a treadmill for 15 min/day for a total of 3 days. There was no difference in N balance and body wt change between exercised and nonexercised animals as well as no difference between animals who experienced different degrees of exercise. Total and individual plasma amino acids in the exercised group were 15% lower than in the nonexercised group; the total and individual free amino acids in muscle of the exercised group were 66% higher than in the nonexercised group. This pattern of high levels of amino acids in the muscle, coupled with decreased plasma concentrations, is suggestive of increased amino acid turnover in the muscle of the exercised animals. In the postoperative or postinjury period, exercise apparently has a stimulating effect on amino acid turnover in the skeletal muscle resulting from amino acid (mainly the branched-chain amino acids) consumption for energy metabolism and gluconeogenesis and protein synthesis. The net balance of both results in N equilibrium similar to that of a nonexercised control group.