In order to study the process of angiogenesis a free skeletal muscle flap was transplanted onto the myocardium of the dog. A microvascular anastomosis with the internal mammary artery (IMA) provided the blood flow to the free graft. In 6 dogs, a myocardial infarction of the anterior wall was induced in order to produce a tissue injury. Four weeks later the skeletal muscle flap was transplanted onto the area of infarction in two dogs with its pars anterior in contact with the heart. In two dogs, the muscle was grafted with its pars posterior, i.e. the muscle fascia between the heart and the graft. In two other dogs, the flap was transplanted with its pars anterior in contact with the heart onto a healthy myocardium. Two animals with infarction were left as controls. Eighteen weeks later a microcorrosion cast was prepared from the hearts with the muscle flap. This evidenced neoangiogenesis in the heart only after transplantation of the graft with its pars anterior onto the myocardium whether it was ischemic or not. The intact muscle fascia, though, inhibited sprouting of vessels. In the controls no angiogenic activity was noted. Results are discussed with regard to the probable origin of the neovessels.