The healing of gastrocnemius muscle injury induced with a spring-loaded hammer in rats was analysed ultrastructurally and immunohistochemically. The ends of the ruptured myofibers retracted, which resulted in a central blood filled cavity. Central zone (CZ) later becomes occupied by granulation tissue scar. CZ is surrounded by a zone where myofibers are necrotized and phagocytosed by days 2-3. Complete regeneration within the preserved basal lamina (BL) cylinders takes place in 5-7 days. The regeneration of myofibers across the scar follows a pattern different from that within BL cylinders. Thin, often branched, myotubes grow out of the BL cylinders into the granulation tissue, where they extend between fibroblasts and collagen fibrils, in general oriented parallel to the preserved myofibers. The extension of the regenerating myotube seems to take place primarily by local synthesis in the growth cone. Fusing myoblasts provide nuclei, mostly along the sides but also occasionally at the tip of myotubes. Some myoblasts seem to derive from undifferentiated cells in the granulation tissue. By three weeks only a few thin myotubes had extended across the gap between the stumps of the ruptured myofibers. Growing myotubes appear to attach to the surrounding connective tissue by specialized structures, an attachment which evidently transmits contraction force across the gap, allowing use of the injured limb before the healing is complete.