The effects of the active metabolite (18-502) of bopindolol, which is a new nonselective beta-adrenoceptor antagonist, were studied on the ischemic changes in myocardial segment shortening, cardiac lactate metabolism and S-T segment of subendocardial electrocardiogram during coronary stenosis and on their recoveries after reperfusion in anesthetized dogs, and were compared with those of propranolol at a dose exhibiting a comparable degree of beta 1-blocking activity. In the presence of coronary stenosis, intravenous administration of 18-502 (5 micrograms/kg) and propranolol (0.2 mg/kg), but not saline, produced significant improvements of regional myocardial dysfunction, lactate production and S-T segment elevations in the ischemic myocardium, which were associated with significant decreases in heart rate and cardiac contractility. After release of the stenosis, administration of 18-502, but not propranolol, resulted in a significantly accelerated recovery of the ischemic segment function as compared with the control group. In rat heart homogenates, 18-502 inhibited the lipid peroxidation approximately 4 times more potently than propranolol. These data show that 18-502 exerts favorable effects during myocardial ischemia produced by coronary stenosis and that it has a cardioprotective action against the contractile dysfunction following reperfusion.