Microvascular anastomosis is gaining increasing significance in the field of replantation and reconstructive surgery. Although traumatic interference is minimized by microsurgery, thrombotic occlusions remain the major reason for clinical failure. With the improvement of suturing technique studies were directed towards the causes of failure using light and scanning electron microscopy. Our study was focussed on the latter aspect. The left common carotid artery of 55 albino rats was transsected and re-anastomosed by the eccentric biangulation technique. The suture-lines were observed after various postsurgical intervals, ranging from 12 hours up to 14 months. Twelve hours after the anastomosis had been performed, the most striking alterations of the endothelium were not found at the suture line, as was expected, but in those parts of the vessels, which were caught by the clamps. The injury leads to sloughing of the endothelium in extensive areas. The exposed subendothelial layer of connective tissue is covered with aggregates of thrombocytes. The suture itself does not lead to thrombus formation. After 14 days, healing is almost complete with total regeneration of the endothelium. From the 21st postoperative day on, the inner lining can no longer be differentiated from that found in experimental animals after 14 months.