A study was made of the response of adult newt ventricle to injury. The major events in the repair process after wounding involve blood clot formation, coagulation necrosis, macrophagic activity, regenerative activity of heart muscle, and connective tissue formation. Light microscopic autoradiography indicated that there was a definite area of labeled cells in trabeculae adjacent to the wound, beginning at about ten days after injury. Electron microscopic autoradiography confirmed that myocytes containing myofibrillae underwent DNA synthesis. It was concluded that adult newt myocardium is capable of reacting to injury by the proliferation of myocytes at the wound area as indicated by both DNA synthesis and mitosis.