Myonuclei and satellite cell nuclei were differentially labelled with 3H-thymidine in uninjured skeletal muscle of young rats and then traced autoradiographically at intervals after mincing the radioactive hindlimb muscles to determine the source of regenerating presumptive myoblasts. Labelled nuclei were detected by light microscopic examination of 1-m̈ thick autoradiographs and identified by electron microscopic examination of an adjacent section. Repeated injections of 3H-thymidine during fetal and neonatal development, followed by a 4- to 5-week maturation period, resulted in labelling of 20% of the myonuclei. Satellite cells were not observed to be labelled in this series. Eight to sixteen hours after mincing, 20% of the pyknotic myonuclei were labelled, whereas none of the regenerating presumptive myoblasts appeared labelled. A single injection of 3H-thymidine administered to 18-day-old rats, followed by sacrifice within ten hours, resulted in labelling of 23% of the satellite cell nuclei. Myonuclei were not observed to be labelled in this series. Eight to sixteen hours after mincing, silver grains were detected over both pyknotic and regenerating cell nuclei. These experiments indicate that many satellite cells survive muscle injury and transplantation to become regenerating myogenic cells at a time when most, if not all, myonuclei are undergoing pyknosis.