Predilection of the human abdominal aorta for developing atherosclerosis in contrast to the thoracic segment is a well recognized fact. A similar phenomenon has also been demonstrated in the dog aorta in experimentally induced atheromatous lesions.1-4While mechanical factors have been advanced for explaining the difference between these two aortic segments, our previous studies on the fate of canine aortic homografts suggested that a biologic difference may account for it. Indeed, in previously reported experiments,3,4fresh thoracic and abdominal aortic homografts were implanted into the abdominal aorta of dogs subsequently subjected to a chronic atherogenic regimen. The thoracic implants exhibited minimal or no atheromatous changes in contrast to the abdominal implants which always developed marked lesions. From these studies it appeared that the aortic segments maintained their original behavior, namely, greater susceptibility to atherosclerosis of the abdominal as compared to the thoracic segment. To test further this concept