These data would suggest that hypothermia combined with potassium cardioplegia enhances protection of the ischemic myocardium over other available techniques. The ideal conduct of this myocardial protection is not yet apparent but certain aspects are worthy of emphasis. (1) With the onset of ischemia cardioplegia should be immediately induced to abolish contractile activity and conserve energy. An advantage of blood cardioplegia is that there is no ischemia or it is trivial priorto cardioplegia. (2) The greater the degree of myocardial cooling the better. Although a myocardial temperature of 20 degrees C can commonly be achieved with perfusion hypothermia and topical hypothermia, it is possible to reduce myocardial temperature to 10 degrees C or lower with these same modalities. Because perfusion hypothermia provides fairly uniform rapid myocardial cooling, this should be maximally utilized by cooling of the systemic perfusate to 20 degrees C and cooling the cardioplegic infusate to 4-10 degrees C. Cardiac hypothermia should be maintained with crushed ice made from electrolyte solution or irrigation of the pericardial sac with cold electrolyte solution. The greater the degree of systemic hypothermia the less tendency for the myocardium to warm. (3) The ideal concentration of potassium is unknown at this time with a range of 15-40 mEq/l having been utilized without apparent potassium injury. (4) The ideal composition of the vehicle may never be defined and may not be of great importance. Whole blood would appear to offer physiological and pragmatic advantages over asanguinous vehicles. (5) The safe duration of ischemia has been moderately well defined. 1 h is well tolerated in the dog using profound cardiac hypothermia, whereas 30-45 min with lesser degrees of hypothermia is acceptable. When the interval of ischemia is to be 2 or 3 h reinfusion of potassium every 20-30 min has proven safe both experimentally and clinically.