Skeletal muscle undergoes active remodeling in response to endurance exercise training, and the underlying mechanisms remain to be fully defined. We have recently obtained evidence that voluntary running induces cell cycle gene expression and cell proliferation in mouse plantaris muscles that undergo fast-to-slow fiber switching and angiogenesis following long-term exercise. To ascertain the functional role of cell proliferation in skeletal muscle adaptation, we performed in vivo 5-bromo-2-deoxyuridine (BrdU) pulse-labeling (a single i.p. injection) and showed a phasic increase (5-10 fold) in BrdU-positive cells in plantaris muscle between day 3 and day 14 during 4 weeks of voluntary running. Daily intraperitoneal injection of BrdU for 4 weeks labeled 2.0% and 15.4% of the nuclei in plantaris muscle in sedentary and trained mice, respectively, and revealed myogenic and angiogenic fates for the majority of the proliferative cells. Ablation of the resident stem cell activity by X-ray irradiation did not prevent voluntary running-induced increases of type IIa myofibers and CD31-positive endothelial cells, but completely blocked the increase of muscle mass. These findings suggest that resident stem cell proliferation is not required for exercise-induced IIb-to-IIa fiber type switching and angiogenesis, but required for activity-dependent muscle growth. The origin of the angiogenic cells in this physiological exercise model remains to be determined.