It is recognized that present-day techniques of intracardiacsurgery demand a satisfactory method of elective cardiac arrest. Potassium citrate, acetylcholineand hypoxic arrest have allbeen used with varying degrees of success.Each of these methods has its limitations and with each, difficulties in resuscitationare encountered. In general,because of the greater ease of resuscitation,hypoxic arrest seems preferableto potassium citrateor acetyicholine.Our previous studies’suggest that hypothermia offerssome degree of protection to myocardial metabolism by preserving sources of energy for myocardial contraction. It was demonstrated that when the dog’s heart was permanently arrested at normothermia with potassium citrate, myocardial glycogen values decreased by 78 per cent, ATP by 60 per cent and lactic acid increased by 500 per cent by the end of 90 minutes of arrest. When animals were placed on cardiopulmonary by-pass and arrest produced with potassium citrate or hypoxia for 30 minutes these changes were of the same order. When total body hypothermia was utilized with by-pass, however, with myocardial temperature reduced below 20CC., glycogen values were essentially unaltered and lactic acid increased to a less degree. Adenosine triphosphate and phosphocreatine values were less consistent, but some degree of protection by hypothermia was suggested. These results are similar to those recently presented by Gott.’ Cardiac resuscitation after arrest with potassium citrate or hypoxia has been shown to be poor after periods of 20 minutes. When hypothermia was utilized with arrest, resuscitation was accomplished in all cases with ease. Therefore, these biochemical studies correlated well with the observation that the heart behaves better functionally when hypothermia is used. Since the more complicated cardiac defects often necessitate cardiac arrest for prolonged periods of time, it seemed desirable to study further the protection afforded by hypothermia when used with cardiopulmonary by-pass and hypoxic arrest, and to establish the length of time over which this protective effect might be effective. Methods Mongrel dogs, weighing 12-15 kg., were placed on cardiopulmonary bypass with a Kay-Cross disc oxygenator and a DeBakey roller pump. Right thoracotomy was performed through the fourth intercostal space.