THERE have been previous experimental attempts to transplant hearts from one animal to another. 1 However, these transplants consisted of merely placing a donor heart in some portion of the recipient's vascular system, and at no time did the donor heart perform the function of maintaining complete circulation in the recipient animal. We have been working with hypothermia as a method to allow a direct approach to intracardiac surgery, without the use of a complicated heart-lung apparatus. 2 By the use of hypothermia we have been able to completely stop all circulation for periods up to 30 minutes without subsequent morbidity or mortality. It occurred to us that such a period of time would be more than ample to perform a complete transplantation of the heart—removal of the recipient animal's heart and the substitution by a donor heart. At present we have successfully performed complete transplantation of the heart in