Study on profound hypothermia with aid of extracoporeal circulation with large volume hemodilution using 20cc/Kg of LMWD in 5% dextrose as diluents, was performed. Expermitents were devided into four groups; whole blood group, 20% hemodilution group and 70% hemodilution group. In 50% and 70% group, the perfusion cooling was initiated with 20% diluted blood and at 25℃ of esophagno temparature (E. T.) the perfused blood was partially replaced with diluents, resulting in 50% hemodilution or 70% hemodilution. On rewarming the large volume hemodilution was adjusted to 20% by replacing with fresh heparinized blood at 25℃ of E. T. The results obtained were as follows. 1) Four of 14 cases of 70% hemodilution group were died of pulmonary edema at the end of perfusion. Two of 14 cases survived inlong term. One of 5 cases of 50% hemodilution group survived long term and others of all group were died within 24 hours post perfusion. 2) High grade metabolic acidosis occured in 50% and 70% hemodilution group at 10℃ of E. T. but by replacing with fresh heparinized blood at 25℃ of E. T. was improved. In several cases of 50% and 70% hemodilution group it still continued at the end of perfusion. 3) In all cases arterial PO(2) was kept higer than normal value during the whole perfusion. 4) Serum Na and K concentration were not changed during perfusion with whole blood below 25 C of E. T. 5) Blood pooling volume was increased higher in whole blood group than in hemodilution groups. No significant difference was found among all hemodilution groups. 6) There was no significant change of arterial blood viscosity between 50% and 70 group during perfusion. In whole blood group, viscosity was increased higher on cooling and decreased on rewarming. In 20% hemodilution group, viscosity was increased relatively higher on cooling and decreased to preoperative value on rewarming. 7) In whole blood group blood flow in carotid artery maintained 32% of the preoperative value at 10℃ of E. T. when blood pressure was 39% of the preoperative level. In 70% hemodilution group; the flow and pressure were 42% and 26% of the preoperative values respectively. 8) From the results of the studies optimal hemodilution ratio below 26℃ of E. T. was thought to be between 20% and 50% 9) Clinical study on profound hypothermia were performed in 12 cases by closed or open chest method with 35-40% hemodilution below 25℃ of E, T. Three of 12 cases were not awake after perfusion but in no cases was profound hypothermia ineriminated as being responsible for the death of the patients.