A study was made of the healing of aortic endothelium in rabbits following the production of a defined superficial injury. This was induced using a fine nylon filament which removed the endothelial cells without producing significant damage to underlying structures. The morphology of the injury and subsequent repair was observed using light microscopy and scanning and transmission electron microscopy. Two forms of injury were produced (a) a longitudinal injury along the full length of the aorta which was 50-80 microns wide (about 5-8 cell widths), (b) a circumferential injury approximately 80 microns wide (about 2 cell lengths). Thirty minutes after injury the exposed tissue was almost devoid of adherent cells, but after 4 h became covered by a sparse monolayer of platelets. Occasional leukocytes were also present from 7 h after injury. Injury tracks were found to repair very quickly; re-endothelialisation being complete by 48 h and there being no sign of injury by 7 days.