The many advantages of combining low flow extracorporeal circulation and hypothermia for open heart surgery have been stressed by recent reports from this clinic and laboratory.1-5These studies were concerned with temperatures of 28 to 30 C combined with extracorporeal flow rates of 25 to 50 cc. per kilogram per minute. To meet other clinical situations that might demand either complete or almost complete circulatory standstill for appreciable periods of time, it would be desirable to use temperatures below 20 C. Since the introduction from this clinic of a feasible method of precise control of the body temperature,3,6the clinical application of this degree of hypothermia became possible for the first time. This report outlines the first of our studies in the laboratory on extension of the advantages of hypothermia to these low levels. Methods Adult dogs of both sexes were used. Anesthesia was induced by intravenous pentobarbital