In general, the mobilisation of structural proteins is necessary for enzyme synthesis and for renewing cellular structures with amino acids and precursors of nucleotic acids. However, during exercise the adaptive synthesis of proteins occurs only in the liver to some extent. In muscle tissue most protein synthesis is suppressed, although the synthesis of certain proteins in muscle remains unchanged or even increases. The general suppression of protein synthesis in muscle leaves much of the free amino acid pool unused. The breakdown of tissue proteins may also increase in various tissues, but there is no convincing evidence for proteolysis of contractile proteins in active muscle. As a result of these processes, an increased pool of available free amino acids is created. The main use of free amino acids is connected with the energy requirement of muscular activity, through the oxidation of branched-chain amino acids and the use of alanine in gluconeogenesis. In active muscles the output of alanine is increased. It is based on usage of pyruvate, which is produced in increased amounts due to intensified glycogenolysis and glycolysis, and of amino groups, which are liberated in oxidation of branched-chain amino acids. In the liver, alanine is consumed. The carbon skeleton of alanine is required for gluconeogenesis and the liberated amino groups are used in ureagenesis. The branched-chain amino acids are transported from the liver to active muscle for their oxidation. The increases in the free amino acid pool, in the rate of the glucose-alanine cycle, and in the use of amino acids in the liver are stimulated by an increased level of glucocorticoids and a decreased level of insulin during exercise. During recovery after exercise the use of amino acids for adaptive protein synthesis is intensified. This coincides with a persistently high rate of protein breakdown, constituting an increased rate of protein turnover. During recovery, the production of 3-methylhistidine by previously active muscles increases. It results in an increase in urinary output of 3-methylhistidine after exercise. Immediately after exercise the level of free 3-methylhistidine is elevated in the intestine for only a short time and the fact that it does not contribute significantly to the delayed increase in the excretion of 3-methylhistidine excretion after exercise must be considered as a sign of increased turnover of contractile proteins, helping to restore a good contractile function.(ABSTRACT TRUNCATED AT 400 WORDS)