The healing of conventional microvascular end-to-side anastomoses has been reported in several studies. The authors studied thrombus formation under scanning electron microscopy (SEM) in an end-to-side anastomotic model in rat carotid arteries. Twenty anastomoses were carried out with 10-0 nylon interrupted sutures. Thrombus formation was evaluated 30 min after clamp release. The findings indicated that the first 30 min are the most critical time for thrombus formation. Thrombi underwent partial lysis or embolization after this time, and gross intraluminal thrombi did not recur. Thrombus formation was studied at four different sites in the end-to-side anastomoses. The sites of needle holes and sutures were primary locations for thrombocyte adhesions and aggregations in normal, vascular anastomoses.