In a previous report from this laboratory,1 it was noted that acute changes in peripheral resistance during cardiopulmonary bypass at normal body temperature, brought about through the administration of vasoconstrictor and vasodilator drugs, were important as indicated by the oxygen uptake of the organism from the perfusate, and by the brain function as measured by the electroencephalogram. Vasodilatation produced by the injection of trimethaphan (Arfonad) camphorsulfonate resulted in a marked narrowing of the oxygen arteriovenous difference, which reflected a diminution in oxygen uptake. This was accompanied by adverse changes in the electroencephalogram. The intravenous administration of epinephrine reversed this effect and restored both the peripheral vascular resistance and the oxygen uptake to its previous level. The utilization of hypothermia as an adjunct to cardiopulmonary bypass introduces various factors into the ordinary relationship that exists between oxygen uptake and peripheral resistance. Some of these factors can be studied by methods