To evaluate the possibility of inadequate right ventricular protection during operation, the temperatures of the anterior myocardium of the right ventricle and the middle of the interventricular septum were compared at ten-minute intervals throughout the period of continuous coronary ischemia in 130 consecutive patients. Systemic temperature was lowered to 23 degrees C, using cardiopulmonary bypass. Cardiac arrest was induced by aortic cross-clamping and infusion of cold cardioplegic solution. Cold solution was reinfused as necessary to maintain septal temperatures at less than 20 degrees C. Despite the use of superior and inferior vena caval cannulation for control of venous return, it was more difficult to maintain the right ventricle at the desired degree of myocardial hypothermia than the left ventricle. The difference between left and right ventricular temperatures was as great as 19 degrees C. In 80% of the observations (n = 1,010), the right ventricle was warmer than the left ventricle. The most frequently occurring temperature differences (left ventricle minus right ventricle) were in the 2 degrees to 3 degrees C range. These data indicate that it is more difficult to maintain hypothermia in the right ventricle. Concern for the left ventricle alone may be misleading. An alarming degree of rewarming may occur in the right ventricle and thereby contribute to right ventricular dysfunction and unilateral right ventricular failure.