Changes of plasma catecholamines during surgery performed in the presence of a combined electrical stimulation analgesia were studied in 11 patients undergoing open heart surgery and in 5 patients undergoing abdominal operations. Stimulation was applied through 2 needles attached subcutaneously to each ear. In 8 of the 11 patients undergoing open heart surgery an up to 20 fold increase in the level of adrenaline in plasma was observed intraoperatively indicating a massive activation of the adrenal medulla. Adrenaline remained elevated also in the postoperative period. The level of noradrenaline did not increase either during the intra- or during the postoperative period. In contrast no changes in adrenaline or noradrenaline could be demonstrated in the remaining 3 patients in whom the concentration of the catecholamines was in the lower normal range. In 5 patients with abdominal surgery noradrenaline was elevated additionally to the increase of adrenaline in the intra- and postoperative period. The increase in the activity of the sympathetic nervous system was associated in these patients with an increase in heart rate and systolic as well as diastolic blood pressure. On the basis of these results the use of a combined electrical stimulation analgesia for abdominal surgery in patients at risk and for open heart surgery is questioned.