In this review we will show that resting SM actively participates in the handling and metabolism of amino acids in the overnight fasted state and following ingestion of a protein-containing meal, and that muscle actively collaborates and exchanges amino acids with other tissues. Major and rapid: changes occur in the muscle free amino acid pool during exercise. Evidence will be presented to indicate that changes in the size of the muscle pool of some amino acids play an important role in the establishment and mainte-nance of a high concentration of tricarboxylic acid-(TCA) cycle intermed ates. This mechanism then allows for the maintenance of a high TCA-cycle flux and aerobic oxidation rate during prolonged exercise Amino aids also seem to play a role in the failure to maintain high concentrations of