This study determines whether reperfusion of the heart with elevated blood levels of epinephrine (E) and norepinephrine (NE) during cardiac surgery produces deleterious effects. The study was conducted in 60 patients undergoing coronary artery bypass surgery. Arterial catecholamine values increased significantly (p < 0.05), from prebypass control levels of 152 ± 29 and 327 ± 30 pg/ml of E and NE, respectively, to 415 ± 78 and 554 ± 49 pg/ml, at initiation of perfusion of the heart after the aortic cross-clamp was removed. Serial measurement of arterial (A) and coronary sinus (CS) E, NE, potassium, lactate, PO2 and CK-MB revealed that during 10 minutes of reperfusion the heart extracted E (positive A-CS difference, p < 0.05), but that the NE A-CS difference was 0. The CS effluent contained significantly (p < 0.05) higher concentrations of potassium, lactate and CK-MB during reperfusion than before aortic occlusion. There was no significant correlation of arterial E and NE, CS E and NE or A-CS differences in E and NE with myocardial release of lactate, potassium or CK-MB. There was a weak association (r = 0.4, p < 0.01) between coronary sinus CK-MB and aortic occlusion time. Maximal arterial E and NE values did not correlate with 10-hour postoperative (maximal) CK-MB values. These results indicate that reperfusion of the postarrested ischemic heart with high levels of endogenously released catecholamines does not worsen ischemia or contribute significantly to myocardial damage. © 1983.