Increasing use of arterial grafts for segmental arteriosclerosis obliterans and arteriosclerotic aneurysms has brought the question of their long-term fate into focus from two angles: (1) their biologic integration into the host, and (2) their susceptibility to the host's atherogenic factors. An answer to these questions would obviously involve long-term clinical observations. Data so far available on this subject are scant.1A more comprehensive analysis of these factors may be provided, however, by animal experimentation. The present study was undertaken to investigate by histological and biochemical methods the fate of fresh thoracic aortic implants into the abdominal aorta of dogs subjected to a chronic atherogenic regimen. Material and Methods Fifteen male mongrel dogs, ranging in age from 1 to 3 years and weighing between 11 and 19 kg., were used. Sterile homologous segments of the lower thoracic aorta freshly secured were implanted within one to two hours into the