In a previous investigation1 it was shown that fresh homologous thoracic aortic implants into the abdominal aorta of dogs receiving a chronic atherogenic diet exhibited minimal or no atheromatous changes, while extensive lesions were found in the host abdominal aorta. It appeared from this study that fresh thoracic homografts showed far less susceptibility to atherosclerosis than the host abdominal aorta. These findings, coupled with our previous study showing a metabolic difference between thoracic and abdominal aorta in the dog,2,3 suggested the existence of a biologic dissimilarity between the two aortic segments. To test this hypothesis further, another series of experiments was undertaken in which fresh abdominal aortic homografts were implanted into the abdominal aorta. The present study is concerned with the fate of these grafts investigated by histological and biochemical methods. Material and Methods Fourteen mongrel dogs, ranging in age from 1 to 3 years and weighing between