This study was undertaken to evaluate the optimal myocardial temperature during a 4-hour ischemia induced by blood potassium cardioplegia (PC). Heart-lung preparations with work load circuit were used as experimental model. 24 mongrel dogs were divided into four groups and two myocardial temperatures (moderate hypothermia, deep hypothermia) and two perfusates (PC with blood, PC without blood) were tested. Parameters used to evaluate the myocardial protection include the coronary sinus blood pH of the initial reperfusion and left ventricular stroke work after resuscitation. In blood PC at moderate hypothermia, deleterious acid metabolites were minimally produced as evidenced by the coronary sinus blood pH (7.221 +/- 0.122) at initial reperfusion, and cardiac function was best recovered in this group. Blood PC at moderate hypothermia allows the heart to tolerate 4 h ischemia without an excessively elevated coronary perfusion pressure during cardioplegic infusion.