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The changes of plasma catecholamins and serum amino acids values at single-level exercise were assessed at the intensity of ventilatory threshold (VT) in nine healthy middle-aged male subjects, Graded exercise tests were performed for individual volunteers in order to detect VT. One bout of treadmill exercise was carried out by each volunteer for 20 minutes with warming-up and cooling-down practices. The intensity of exercise was determined by oxygen consumption(Vo₂) which was prescribed by the formula of American College of Sports Medicine. Blood samples were obtained from the cubital vein at the initial rest and just after exercise. Mean VT value was 24.9±3.3 ml･min⁻¹･kg⁻¹. Plasma glucose concentration decreased significantly, but blood lactate value did not exceed lactate threshold(LT ; blood lactate concentration which reaches a fixed value of 2 mM). Total amino acid and alanine tended to increase slightly, But there were no significant change in branched-chain amino acids values. However, plasma catecholamines contents increased significantly (adrenaline;66%, noradrenaline;45%). It is concluded that treadmill exercise at VT level of Vo₂ ｃan be utilized effectively for therapeutic exercise. Further investigation should be done for the detailed fluctuation of catecholamines during therapeutic exercise., Article, 紀要 20: 61-69(1995)