Myocardial infarcts were artificially induced in a series of dogs by ligation of the circumflex coronary artery. Infarcted areas were studied by transmission electron microscopy at varying times between 3 1/2 days to 126 days post-ligation. A granulation phase was demonstrated up to 13 days post-ligation, but fibrin and inflammatory cells were not usually seen after that. The fibroblasts showed increased rough endoplasmic reticulum and Golgi vesicles up to 30 days. Beyond that they reverted to a more quiescent morphology. Myofibroblasts were abundant at 13 days but were not found beyond 30 days. Many microvessels were occluded by excessive numbers of endothelial cells from 22 to 30 days. Beyond this period they, again, became patent. Many similarities exist between the healing stages of myocardial infarctions of dogs and deep injury to the body surface in man. It is suggested that animals form the equivalent of hypertrophic scar or keloid, but resolution of hypertrophy, or the self-limiting process, is much more rapid and efficient in animals than in man.