Atrial homografts from newborn hamsters, transplanted into cheek pouches of adult Syrian golden hamsters, were studied using intravital and electron microscopy. Half of the transplants survived up to 1 year, performing spontaneous pulsatile activity. The transplants could be influenced by β-receptor activating or blocking substances applied intravenously to the host animal. For morphological analysis homografts were taken 3, 6, and 12 months after transplantation. Irregular arrangement of the myofibers occurred, and the T system was seen to be replaced by plasmalemmal and sub-plasmalemmal vesiculations. There was some evidence for hypoxia in the grafts. Dedifferentiation of atrial cardiocytes, somehow resembling smooth muscle cells, is discussed. Despite these morphological changes the specific contractile function of the grafts was preserved.