Deaths from uncontrollable hemorrhage might be prevented by arresting the circulation under protective hypothermia to allow resuscitative surgery to repair these injuries in a bloodless field. We have shown previously that in hemorrhagic shock, circulatory arrest of 60 minutes under deep hypothermia (tympanic membrane temperature, Ttm = 15 degrees C) was the maximum duration of arrest that allowed normal brain recovery. We hypothesize that profound cerebral hypothermia (Ttm less than 10 degrees C) could extend the duration of safe circulatory arrest. In pilot experiments, we found that the cardiopulmonary system did not tolerate arrest at a core (esophageal) temperature (Tes) of less than 10 degrees C. Twenty-two dogs underwent 30-minute hemorrhagic shock (mean arterial pressure 40 mm Hg), rapid cooling by cardiopulmonary bypass (CPB), blood washout to a hematocrit of less than 10%, and circulatory arrest of 2 hours. In deep hypothermia group 1 (n = 10), Ttm was maintained at 15 degrees C during arrest. In profound hypothermia group 2 (n = 12), during cooling with CPB, the head was immersed in ice water, which decreased Ttm to 4 degrees-7 degrees C. The Tes was 10 degrees C in all dogs during arrest. Reperfusion and rewarming were by CPB for 2 hours. Controlled ventilation was to 24 hours, intensive care to 72 hours. In the 20 dogs that followed protocol, best neurologic deficit scores (0% = normal, 100% = brain death) at 24-72 hours were 23% +/- 19% in group 1 and 12% +/- 8% in group 2 (p = 0.15). Overall performance categories and histologic damage scores were significantly better in group 2 (p = 0.04 and p less than 0.001, respectively). We conclude that profound cerebral hypothermia with CPB plus ice water immersion of the head can extend the brain's tolerance of therapeutic circulatory arrest beyond that achieved with deep hypothermia.