The authors have developed an organ culture system in which segments of rabbit aorta were used for the study of the healing of superficial, intimal injury analogous to that produced in vivo by a nylon monofilament catheter. Aortic segments containing an intact endothelial monolayer were obtained for culture by careful avoidance of the damaging consequences of loss of vascular pressure, vasocontraction, and thrombosis. The segment could be maintained in culture for several weeks. Repair of superficial injury was studied after placing a series of precisely located injuries, 100 mu wide, on each aortic segment over a period of several days. The pattern of healing observed was similar to that observed in vivo. The wounded area was recovered by migrating cells within 24 hours, and during succeeding days these cells underwent a predictable pattern of remodeling which restored the normal morphologic features of the endothelium. The cells within the wound underwent a burst of cell division between 24 and 96 hours after injury and subsequently became quiescent. This organ culture approach potentially allows the investigation of many consequences of endothelial injury in a system which provides far greater control and manipulation of the cellular environment than is possible in vivo.