Intracellular amino acids in skeletal muscle show a specific concentration pattern on the third post-operative day. The temporal development of these changes has not been clarified. Here the amino acid concentrations in skeletal muscle were studied during the first post-operative day in fourteen patients undergoing elective abdominal surgery. Muscle amino acids were determined pre-operatively, as well as at 12 and 24 h post-operatively. In muscle the concentrations of glutamine and the basic amino acids decreased gradually during the first 24 h after surgery to 79% (P less than 0.001) and 67% (P less than .001) respectively. The sum of the essential amino acids decreased to 73% (P less than 0.001) at 12 h, but thereafter rose to 91% (P less than 0.05) at 24 h. The sum of the BCAA decreased to 84% (P less than 0.05) at 12 h but then increased to 116% (P less than 0.05) at 24 h. The alanine concentration increased to 122% (P less than 0.001) during the first post-operative day. In plasma the alanine concentration increased at 12 h while most other amino acids declined. At 24 h post-operatively the plasma concentrations of all amino acids had returned to normal or showed a tendency towards normalization except for phenylalanine, which increased. At the end of the first post-operative day the concentrations of amino acids in muscle were consistent with the alterations previously observed three days after surgery. The changes in plasma amino acid concentrations only partly reflected those in muscle.