Repair to injury of the rabbit aorta was studied with special attention to the source of new intimal cells. Two types of injury, suture placement and electrocautery, were used to ascertain whether the type of injury significantly modifies the reparative response of the artery. Cell morphology and DNA synthesis, as determined by autoradiographic localization of tritiated thymidine uptake, were studied sequentially during the course of the repair reaction. Suture injury was the more reliable model for these studies because the magnitude of injury was quite comparable between animals, while with cauterization there was considerable variation in the magnitude of injury. The overall reparative response with the two types of injury was, however, similar. Cell division was first seen in the media adjacent to the injured focus and was followed by migration of medial cells into the intima. In the intima, medial cells continued to divide for a few days and then gradually matured from undifferentiated ovoid cells resembling monocytes to morphologically differentiated smooth muscle cells. Medial cells exhibiting DNA synthesis were both differentiated and undifferentiated smooth muscle cells, supporting previous reports that mature as well as undifferentiated smooth muscle cells are capable of mitosis. However, with injury and repair, the migrating cells appeared to be mainly undifferentiated since there was a predominance of these cells early in the repair reaction in the intima.