The advantages of a bloodless field and total cardiac relaxation have popularized the technique of deep hypothermia and total circulatory arrest for the correction of complex congenital cardiac defects in infancy. There is, however, a significant potential for cerebral and pulmonary complications. Presently, the most common technique is that of using a combination of surface cooling and cardiopulmonary bypass cooling and rewarming. Normal neurological development has been claimed with the present technique of hypothermia at 20 degrees C and total circulatory arrest for periods up to an hour; however, there are reports of seizure activity in the early postoperative period. There is also a disturbing incidence of respiratory insufficiency and, occasionally, hemorrhagic pulmonary edema. This study, using growing puppies and subjecting them to deep hypothermia and total circulatory arrest for varying periods of time, disclosed that animals subjected to 60 min of circulatory arrest recovered neurologically; however, there were histological changes of anoxia in the brain. Animals subjected to 30 min of total circulatory arrest were normal neurologically and there was no histological evidence of anoxic damage to brain tissue. Puppies that were continuously on cardiopulmonary bypass had no significant pulmonary changes caused by increasing the inspired oxygen tension in the ventilator; however, striking changes were noted when limited cardiopulmonary bypass was employed for core cooling and total circulatory arrest combined with pulmonary ventilation with 100% oxygen. We conclude from this experimental study that the use of surface cooling and core cooling with subsequent total circulatory arrest at 20 degrees C is a safe procedure, providing the period of time of cardiac arrest is kept around 30 min. We also conclude that the alveolar oxygen tension should be maintained at the lowest level possible during the interval of circulatory arrest to avoid the apparent rapid onset of post-traumatic pulmonary insufficiency.