The aortic endothelium in weanling swine was rubbed ten times with an inflated balloon catheter in a repeated balloon denudation. This procedure produced more drastic, extensive and uniform changes in the aortic wall than the commonly used balloon denudation. Sequential alternations of regenerating endothelium, intimal thickening and medial reaction, and the incidence of mitotic cells were studied by scanning and transmission electron microscopy. Complete endothelial loss was confirmed from the descending thoracic aorta to the right femoral artery within 24 hr. By the third day, regenerated endothelium began to cover over the endothelial defect area from the uninjured areas such as the aortic arch and the orifices of branching arteries. Thrombus formation and fibromuscular intimal thickening were observed in the endothelial defect areas by the fifth and seventh days. Three types of mitotic cells, such as endothelial, intimal and medial cells were noted in the aortic wall. Mitotic endothelial and mitotic medial cells were most frequent at Day 3; the latter were closely associated with dead medial cells. Mitotic intimal cells initially appeared at Day 3 and were most frequent at Day 7. Mitotic intimal and mitotic medial cells were frequently present in the aortic wall subjacent to the endothelial defect areas containing interstitial blood components. The large numbers of mitotic aortic cells indicated that endothelial cells give rise to new endothelial cells, intimal cells to new intimal cells, and medial cells to new medial cells.