Objective:To study the protecive effect of ultraprofound hypothermia on canine ischemic brain. Methods:Seven adult dogs underwent closed-chest extracorporeal circulation with both external and internal body cooling. As they were cooled, progressive hemodilution with Haemaccel was employed until the hematocrit decreased to 5%or so. Continuous circulation and a core temperature at 7-10℃ were maintained for 90 min. During rewarming, the animals were autotransfused and allowed to recover. Results:Of the 7 animals, one died intraoperatively due to technical factors related to closed-chest extracorporeal circulation, 6 surviving animals showed normal neurological functions, but died of dysfunction of lungs 2-4 h after resuscitation. Pathological examination revealed mild ischemic lesions of myopcardiac structure, pulmonary edema with microscopic alveolar hemorrhage and no histological abnormality of brain, liver or kidney. Conclusion:The canine brain, liver and kidney can tolerate ischemia for 90 min under ultraprofound hypothermia, despite the dysfunction of lungs after resuscitation.