Deep hypothermia (20 C) without cardiopulmonary bypass is a valuable technique during cardiac surgery in infants but rewarming of the heart following circulatory arrest and cardiac repair has traditionally been a lengthy and difficult process. In experimental animals rewarming the heart with microwave energy, as reported in this work, warms the heart before warming the periphery. In 18 mongrel dogs that were surface cooled to 20 C, we found that during microwave rewarming the core temperature rose 4.7 C per hour. Whole body oxygen consumption, heart rate, and cardiac output returned to normal at rates equal to the rates at which they decreased during surface cooling. Blood pressure and arterial gases remained adequate. Microwave rewarming appears to be a useful method for reestablishment of cardiac function and normothermia following deep hypothermia.