Amino acid responses to exercise stress in well-trained racehorses and human athletes are well characterised, but the knowledge of amino acid metabolism during and after exercise in inactive animal species is limited. To study this, plasma amino acid concentrations were measured in previously unexercised pigs which performed two exercise tests on a treadmill with an interval of one week. In general, the changes in amino acids were more pronounced after the second than after the first exercise bout. Alanine, glutamine, phenylalanine and tyrosine were elevated for one hour only after the latter exercise. Twenty-four hours after the second exercise isoleucine, leucine, phenylalanine, tyrosine and valine were increased, but only isoleucine was increased after the first test. These differences between the two tests might be explained by adaptation of the amino acid metabolism after a single exercise bout and suggest that domestic pigs are well suited to study the early effects of exercise.