The time course and pattern of the initiation of satellite cell mitoses was studied following a crush injury of the distal third of the 30-day-old rat extensor digitorum longus, a muscle in which myofibers run the entire length of the belly. Satellite cell mitotic activity was monitored using autoradiography after injection of 3H-thymidine. Satellite cell labeling rose above control values by 15 hours post injury at the injured site and by 20 hours in the middle third or adjacent undamaged portion of the muscle. Labeling decreased in the proximal third of the muscle, so that by 25 hours post injury, a gradient of labeling was established along the muscle length. Electron microscopy (EM) quantitation showed that the reduced labeling in the proximal third resulted from a reduction in satellite cell numbers. The results suggest that mitotic activity of satelite cells after localized injury occurs mainly at or near the site of injury, but that many of the dividing cells have migrated from undamaged areas distant from the lesion site.