An autoradiographic study has been made of mouse femora from birth to one year of age. Tritiated thymidine was used to study the proliferative rate of various skeletal tissues. The labeling index of the periosteum was found to be highest at birth (0.085). At 8 weeks of age the index fell sharply to 0.007 and remained low throughout the entire period of study. Chondro-osteogenic cells of the periosteum, especially those at the perichondrial zone, were most frequently labeled initially. The labeling index of the epiphyseal disk remained high (0.054 to 0.048) until about the 6th week of age. Following this age the index fell sharply to 0.018 at 8 weeks and 0.002 at 26 weeks of age. Autoradiographs showed that the cells of the articulating surface of the epiphysis and disk are derived at least in part from migrating chondro-osteogenic cells of the periosteum residing at the perichondrial region, and that these cells serve as the progenitor pool necessary for both circumferential growth and expansion of the epiphyseal disk.