The time course and extent of satellite cell activation were studied in the soleus (m-SOL) and extensor digitorum longus (m-EDL) muscles of untrained growing and mature rats after a single bout of prolonged eccentric treadmill running. At 24, 48, 72, and 120 h postexercise, satellite cell mitotic activity was quantitated in autoradiographs of whole-fiber segments after injection of [3H]thymidine. Fiber damage and localization of labeled cells were also examined in muscle cross sections. Labeling in growing muscles progressively increased to peak levels (approximately 250% of control) at 72 h postexercise, whereas mature muscles exhibited an earlier peak (approximately 250% of control) at 24 (m-SOL) and 48 (m-EDL) h, followed by a more rapid decline to control levels by 120 h postexercise. In all exercised muscles the calculated satellite cell activation was far greater than required to repair the small number (less than 3.0%) of necrotic fibers identified at the light-microscopic level. These results suggest that satellite cells were activated not only on fibers exhibiting overt necrosis but also on those with lesions not discernible with light microscopy.