We attempted to evaluate the effectiveness of cold blood potassium cardioplegia [Group B] compared with that of intermittent aortic cross clamp with topical hypothermia [Group A] in each six dogs. The studies were performed under the extracorporeal circulation with moderate hypothermia by using Mongrel dogs. 1. In Group A [6], it was difficult to maintain the temperature below 20?. but in Group B [6], it was possible to maintain the temperature. 2. In blood pressure and myocardial recovery ability, Group B was more excellent than Group A. 3. There was no significant difference in Na+, Ca++ and Cl- between both groups, but in K+ level, Group A was mild decreased and Group B was nearly normal level. 4. In serum LDH level, Group A was higher than Group B, but in SGOT level there was no significance between both groups. 5. On electromicroscopical study, Group A showed severe change in mitochondria, but Group B was nearly normal view. Thus we suggest that cold blood potassium cardioplegia would be more effective than intermittent aortic cross clamp with topical hypothermia for myocardial protection in open heart surgery.