Early endothelial injury may play a role in the development of transplant arteriosclerosis. The present study documents early endothelial changes using a rat aortic graft model. Abdominal aortic allografts from PVG rats were orthotopically transplanted to DA rats. Controls were DA to DA transplants. Endothelial cell (EC) injury, regeneration, and leukocyte infiltration in the intima were evaluated using scanning electron microscopy and histological and immunocytochemical techniques. Nontransplanted aortic segments showed partial loss of ECs after 1 or 2 hr of preservation. Control isografts demonstrated extensive EC denudation and neutrophil adherence to residual ECs at 1 day post-transplantation. After 3 days, isografts showed continued regeneration of ECs in the central area and ingrowth of endothelium from both clamped sites in the recipient aorta. Reendothelialization was complete by day 14. Allografts showed similar findings to isografts up to day 3. In contrast to isografts, however, there was a secondary EC loss beginning at day 7. Monocytes/macrophages and T cells were noted to be adherent to residual ECs in 7- and 14-day allografts. At 20 days, ECs were absent from the luminal surface in the center of allografts. Endothelium did extend from clamped sites toward the midgraft region as in isografts. By 60 days allografts were completely reendothelialized. These results demonstrate that in both isografts and allografts there is initial EC loss due to mechanical trauma and ischemia/reperfusion injury, followed by partial reendothelialization. This latter process continues unabated in isografts, whereas in allografts the secondary EC loss occurs due to an allogenic response. This is followed by complete reendothelialization that occurs during the concurrent development of significant intimal hyperplasia.