A new myocardial support system has been developed and a study of this system is reported. Cardioplegia was induced by continuous infusion of potent "cardioplegic" agents (potassium chloride and potassium chloride with propranolol) at the aortic root in 12 dogs subjected to cardiopulmonary bypass with total body hypothermia (20 degrees C). A low-flow normal-pressure perfusion was maintained with the aid of a norepinephrine drip. During the period of hypothermia the blood pH was maintained at 7.6 and serum magnesium concentration was increased to an average of 2.1 mmol/l by parenteral infusion of magnesium sulfate. At no time was ischemia induced and the aorta was not cross-clamped. The functional recovery to normal and preservation of the ultrastructure of the subendocardium after 4 hours of perfusion in cardioplegia were remarkable. The control hearts from hypothermic dogs that were allowed to fibrillate spontaneously showed severe damage.