A study has been made of the healing of a narrow deep injury to rabbit aortic endothelium which also involves damage to the media of the vessel. The injury was produced using a nylon catheter containing a wire filament; the injury was approximately 150 micron in width and damaged up to 3 elastic lamellae. Immediately after injury platelet aggregates were observed over the injured areas, several hours later large numbers of leukocytes were also seen to adhere. Two days after injury a non-thrombogenic neointimal surface was observed over deeply injured areas; endothelial cells could be identified covering the injured area at 6 days. The healing process following the injury has been directly compared with the healing of rabbit aortic endothelium following a superficial injury of similar width, where endothelial cells are removed without significant damage to the media of the vessel [1]. The results show that (a) following a narrow injury to the aorta which causes damage to the media platelet aggregation and proliferation of smooth muscle cells occurs, (b) despite the disruption of subendothelial components, endothelium rapidly regenerates over the narrow injured area, although not as quickly as for a superficial injury.