DURING experiments with various cooling techniques for induction of deep hypothermic circulatory arrest, it was noted that certain of the experimental animals exhibited some of the signs of cerebral edema during the rewarming and postrewarming phase.1 Since the appearance of these signs was coincident with the existence of metabolic acidosis of relatively severe degree, a relationship between acidemia, per se, and cerebral edema seemed possible. Such a correlation would be of considerable interest, especially since a relationship between cerebral edema and respiratory acidosis has sometimes been implied, and because metabolic acidosis is usually encountered whenever prolonged cardiopulmonary bypass is used. The experiments alluded to above did involve cardiopulmonary bypass and core cooling in dogs, with or without the addition of external cooling in a water bath. Those animals were brought to temperatures lower than 10 C (50 F) and then subjected to periods of circulatory arrest of up to