A experimental study of deep hypothermia for open heart surgery in 5 dogs was reviewed. Surface hypothermia in combination with limited cardiopulmonary bypass was employed. Circulatory dynamics were well maintained following cardiac arrest during one hour at 20?. Some degree of acidosis usually developed after the arrest period but was gradually corrected during. rewarming Total circulatory occlusion could be maintained for at least one hour at 20? without evidence of cerebral damage in the dogs. Potassium in the serum after deep hypothermia was moderately decreased and it was the most severe change in the electrolyte. Free hemoglobin of serum was mild increased and it was one of advantages of combined hypothermia with limited cardiopulmonary bypass.[KTCS 1977;1:124-132]