The hearts of the 49 adult male rats were injured by the application of electro-coagulator, and the animals were killed at intervals from 12 hours to 2 months after injury.Another 8 rats were used for control. The regenerative changes during the process ofwound-healing were studied with histological method. Two to twenty days after injury, the regenerative changes in myocardial fibers wereevident. The modes of regeneration were shown on several ways. (1) Two days after injury, at the border of the lesion, the terminations of somesurviving myocardial fibers showed variable degrees of dedifferentiation, the nuclei swelledup and the nucleoli increased in number and volume. Many of the nuclei were arrangedin pairs, especially in the papillary muscles. (2) Two to twenty days after injury, some mitotic figures were found in myo-cardial fibers. (3) Five to nine days after injury, there were many myoblasts in the center ofthe lesion along the necrotic fibers. For lack of favorable conditions, the myoblasts and the terminations of the survivingfibers could not continue to develope and differentiate. The lesion was repaired by thecicatrization.