It is believed that catecholamine secretion is increased during cardiopulmonary bypass. However, the periods of maximum increase in catecholamine levels during cardiopulmonary bypass are different among several authors. 15 patients having valvular surgery were studied. Plasma epinephrine and norepine- phrine were determined by high performance liquid chromatography at 8 stages of the operation. During bypass plasma catecholamine levels continued to rise and maximally increased until aortic cross clamp off, and decreased gradually. Norepinephrine also showed the same results initially, but then increased gradually after the end of bypass. During bypass the maximum increases in epinephrine and norepinephrine were sixfold and twofold respectively in comparison with the levels prior to induction, which suggests that the predominant humoral response to cardiopulmonary bypass appears to be adrenomedullary release of epinephrine. The catecholamine levels at the period of aortic cross clamp off was higher than that of the lowest body temperature. There was no correlation between the increases in catecholamines and mean arterial pressure. Temperautures and eatecholamines also showed no correlations.