Following animal experiments, intraoperative protection of the myocardium was used clinically. It consisted of an initial cardiac arrest, according to the method of Kirsch, and subsequent aerobic cardioplegic coronary perfusion in hypothermia lasting 6 to 8 minutes until a myocardial temperature of 20° C. or below was reached. Oxidative metabolism was maintained by the cardioplegic perfusate used. Cardioplegia, stabilizing of membranes, and hypothermia at 20° C. reduce the energy requirements of the myocardium to 1 to 2 percent of normal. At the onset of the subsequent phase of ischemia, stores of energy-rich phosphate compounds were normal. Under these clinical conditions at 20° C., the tolerance for ischemia lasted for 120 minutes, since t-ATP was reached only at this time. This period of ischemia was long enough for most of the open-heart procedures. Out of 445 patients operated on by means of this myocardial protection method, 10 died, three of them because of technical errors made when this method was still new (overdistension). One death was caused by insufficient myocardial protection. Investigations into postoperative levels of the heart-specific creatine kinase isoenzyme CK-MB revealed no ischemic lesion of the myocardium. The procedure requires strict maintenance of some methodological preconditions, such as keeping the initial ischemic phase as short as possible, limiting the pressure of perfusion to 20 to 30 mm. Hg, controlling myocardial temperature, and preventing overdistension of the paralyzed ventricles. Measured against the rate of postoperative low-output syndrome and the ensuing mortality rate, as well as the number of postoperative infarctions following aorta-coronary artery bypass operations, hypothermic cardioplegic coronary perfusion with initial cardiac arrest, according to the method of Kirsch, in our experience is superior to hypothermal ischemia, intermittent reperfusion, and coronary perfusion of the beating or fibrillating heart.