Murine hearts were “free grafted‘’ to the pinna of adult mouse ears in order to evaluate immunological and physiological factors influencing the take, survival, and function of myocardial grafts in inbred mice. Visible pulsations, electrical activity, and histology were used to ascertain the viability of the grafted hearts. Cardiac isografts from fetal donors showed slightly better survival (72 of 95) than did those from newborn donors (27 of 40). Twenty-two fetal cardiac isografts in the CBA strain showed 100% survival. Some isografts and parent to F1semiisografts have pulsated for longer than 2 years. Approximately one-half of unsexed isografted hearts underwent a slow late rejection (within 20–80 weeks) by female but not by male recipients, suggesting that the Eichwald-Silmser effect is operative for cardiac transplants as well as for skin grafts. Y factor immunity was not demonstrated in CBA mice, and isografts to either sex were equally successful (22 of 22 at 86 weeks). F1to parent and histoincompatible allogeneic heart grafts were rapidly rejected;H-2-incompatible andH-2-compatible allogeneic grafts were rejected within 2 and 3 weeks, respectively. Electrical changes associated with the rejection of the free grafted hearts were characterized by accelerated rate, decreased voltage, and increased incidence of arrhythmia.