SUMMARYA technique for the in vivo microscopic study of functioning cardiac homografts in Syrian golden hamsters has been developed, and a study of the sequence of microcirculatory and cellular events occurring in these grafts is reported. Observations indicate that cardiac grafts are vascularized initially by endothelial buds from the graft tissue itself moving outward and establishing connections with the host vessels. Grafts derived from fetal and newborn donors showed functional activity more frequently than did those derived from adult animals. Similarly, grafts derived from atrial tissue showed contractile activity more often than did ventricular grafts. The hamster cheek pouch technique permits greater resolution of the cellular and microvascular phenomena associated with cardiac graft survival than did previous methods.