Ultrastructural alterations in skeletal muscle fibers were electron microscopically studied in rats forced to run on the tread mill until "all-out". When they were mild and limited to relatively small areas, the reconstruction of filaments ensued within 10 days without infiltration of cells. When they were severe and extensive, phagocytes infiltrated in the lesions and removed degenerative sarcoplasmic debris from muscle fibers. A little later, myoblasts appeared and regeneration was accomplished in 30 days in much the same manner as in myogenesis.