A 3 month program of exercise training was carried out in 10 patients with ischemic heart disease to assess the contribution of catecholamines to the training effect of improved exercise capacity and enhanced myocardlal oxygen delivery. Training resulted in a significant increase in maximal treadmill exercise time (mean ± standard error of the mean 374 ± 28 versus 567 ± 33 seconds, P < 0.0001). The heart rate-arterial pressure product, an index of myocardial oxygen consumption, was decreased at rest and at all levels of exercise after training. After training, the mean plasma norepinephrine level during supine rest was reduced (320 ± 23 to 191 ± 20 pg/ml, P < 0.01) and plasma epinephrine values were unchanged. When these previously sedentary patients exercised on a treadmill to the maximal level attained before exercise training, the mean plasma norepinephrine and plasma epinephrine concentrations were significantly reduced. These data suggest that the beneficial adaptations of the cardiovascular system to exercise training are at least partially mediated by changes in the catecholamine response to exercise.