The methods described have prevented the danger of irreversible ventricular fibrillation in deep hypothermia, and have also permitted complete circulatory arrest combined with exsanguination in experimental dogs for periods of up to 45 minutes. It is possible that the period of 45 minutes of circulatory arrest may be exceeded, but further research will be necessary to confirm this. Smith (1957) observed intestinal haemorrhages in hamsters kept at 0[degree]C for 70 minutes or longer, and this paper also notes observations of acute pancreatitis and intestinal haemorrhage in the dog when the total circulatory arrest has exceeded 60 minutes. These methods have already been successfully applied to patients undergoing open-heart surgery and may also assist in the surgical reconstruction of the aortic arch. In the future when the homograft reaction has been solved, they may prove of value in transplantation of major organs.