Adult, embryonic and tumor tissues from the rat were grafted to the chorioallantoic membrane (CAM) of the chick embryo. Fine structural changes in the microvasculature of the transplants were compared, with emphasis on the condition of endothelial cells. Endothelial cells in tumor grafts appeared irreversibly injured 2 h after grafting and most had degenerated by 8 h. In normal adult tissues, endothelial cells disintegrated more slowly, but lipid accumulation and blebbing leading to occlusion of vascular lumina, were prominent by 1 day. Cells were swollen and vacuolated 2 days after grafting and most were disrupted by 3 days. Embryonic endothelial cells were only mildly damaged and vessel lumina were patent for up to 3 days. Vessels were then reperfused with chick blood. The microvasculature in tumor tissue is extremely susceptible to ischemic damage and this phenomenon may be involved in the central necrosis which develops in many animal tumors after they grow beyond a small size. Furthermore, only blood vessels and endothelium in embryonic tissues were sufficiently resistant to ischemic changes so that the graft microvasculature could be perfused.