The effect of moderate whole-body hypothermia on blood flow to acutely ischemic and nonischemic myocardium and on the relationship between blood flow and necrosis after 5 hours of left anterior descending (LAD) coronary artery occlusion was investigated in 20 dogs. Blood flow to ischemic myocardium was not significantly increased or decreased by hypothermia. However, much less myocardial necrosis (shown by nitro blue tetrazolium staining) was observed in the hypothermia-treated animals. Therefore, this protective effect of hypothermia is not a result of improved blood flow to the ischemic region, but is more likely due to decreased metabolic requirements. Nearby noninfarcted myocardium had slightly higher blood flow than homologous tissue in the corresponding controls under both normothermic and hypothermic conditions. This tissue, which must compensate for the loss of contractility in the ischemic region, appears to maintain its ability to respond to increased demand for blood flow even during moderate hypothermia. Blood pressure and cardiac output data indicate that hypothermia did not interact with myocardial infarction to produce or exacerbate cardiogenic shock. Consequently, whole-body hypothermia may prove to be a safe and effective emergency pretreatment which may significantly decrease the amount of necrotic myocardium when initiated prior to emergency coronary artery bypass surgery.