The cell cycle time of satellite cells in growing rats was determined to be approximately 32 hr, with an S-phase of 14 hr. The estimated cycle time was the same for satellite cells in both oxidative soleus and glycolytic EDL muscles and is consistent with the rate at which myonuclei are produced during growth. Continuous infusion of bromodeoxyuridine (BrdU) was used to determine if all satellite cells had the same cycle timein vivo.Approximately 80% of the satellite cell population was readily labeled over the first 5 days of continuous infusion. Remaining satellite cells accumulated label at a much slower rate and were still not completely saturated after an additional 9 days of infusion. Only a small portion of the cells labeled with BrdU during the first 5 days could be labeled with a second label ([³H]thymidine) during tandem continuous infusion experiments, suggesting that they pass through a limited number of mitotic divisions prior to fusion. These results suggest that satellite cells in growing oxidative and glycolytic skeletal muscles can be subdivided into two distinct compartments. About 80% divide with a 32-hr cell cycle duration and are responsible chiefly for providing myonuclei to growing fibers. The remaining 20% of the cells divide more slowly, probably because the cells enter a G₀-phase between mitotic divisions. These reserve cells, through asymmetric divisions, may generate the myonuclei-producing satellite cell population. Proliferative potential for regeneration and adaptive responses is likely located in this reserve population.