Extra adult rat hearts were transplanted isologously into the abdominal cavities to study their growth reactions. The aorta and pulmonary artery of the donor heart were sutured to the host aorta and inferior vena cava by end-to-side anastomoses, the donor veins being ligated. Such hearts pumped blood from the aorta through the coronary circulation and the right auricle and ventricle. After 2 months to 1 year, about one third of the 62 grafts had undergone atrophy, losing up to two thirds of their original weights at transplantation but retaining nearly normal LV/RV ratios. Of the remaining grafts, 16 had not changed in weight, while 8 had hypertrophied due primarily to the growth of the right ventricular wall. About one quarter of the transplants were grossly abnormal owing to necrosis or osteogenesis.