Thirty-six anesthetized mongrel dogs were subjected to systemic hypothermia and anoxic cardiac arrest while on cardiopulmonary bypass. Myocardial fine structure in the left ventricle was evaluated by quantitative analysis in the subepicardial, mid-myocardial, and subendocardial layers. The specimens were obtained by a transmural myocardial biopsy method. Graded hypothermia was employed at 36 degrees, 30 degrees, 28 degrees, 25 degrees, and 22 degrees C. The aorta was cross-clamped for 30 minutes at 36 degrees, 30 degrees, 28 degrees, and 25 degrees C. or for 45 minutes at 25 degrees and 22 degrees C. Observations indicated that pathological changes of the myocardial fine structure were significant after anoxic arrest in the normothermic group. Lesions were most extensive in the subendocardial layer after normothermic anoxic arrest, whereas hypothermia below 30 degrees C. preserved the myocardium throughout all layers without difference in pathological changes in the fine structure among the layers. Mitochondria and glycogen granules in the mid-myocardial layer and the subendocardial layer were best preserved with hypothermia at 25 degrees and 22 degrees C. after 30 minutes or 45 minutes of anoxic arrest, and dogs in these groups had a survival rate of 100 percent. Quantitative analysis of ultrastructural changes induced in these experiments suggest that a safe duration of anoxic arrest at 22 degrees to 25 degrees C. is between 30 and 45 minutes.