Profound hypothermia with core cooling has been considered unsafe as compared with surface cooling because of the induced metabolic acidosis. We carried out studies on mongrel dogs to determine whether or nor conventional cardiopulmonary bypass with pulsatile blood flow for core cooling could replace the bypass with non-pulsatile flow. The recovery time from anoxic damage of the brain due to circulatory arrest was also studied. Cerebral excess lactate (delta XL) (Huckabee) was determined throughout the course of hypothermia. During the cooling period from 30 degrees C down to 20 degrees C, the mean value of delta XL in the pulsatile group was significantly lower than that in the non-pulsatile group (p less than 0.01). After forty minutes of the first total arrest at 20 degrees C, thirty minutes of pulsatile perfusion tended to eliminate the anaerobic metabolism of the brain caused by the first total circulatory arrest (p less than 0.1). Thus, the anaerobic metabolism in the brain appears to be highly suppressed with pulsatile cardiopulmonary bypass during the cooling period. Correction of congenital heart disease in infancy can probably be more safely performed if pulsatile cardiopulmonary bypass for cooling and rewarming is employed instead of the non-pulsatile bypass.