Regeneration of damaged endothelium has been studied by means of mechanical injury to rabbit abdominal aorta and inferior vena cava. Ultrastructural evidence suggests that myointimal cells can become endothelial cells. It is postulated that such a myogenic replacement of endothelial cells may occur under normal circumstances in the aorta. Injury to blood vessels merely accelerates this process.ADDITIONAL KEY WORDS blood vessel repair myointimal cell