The kinetics of osteoblasts and osteocytes on appositional and resorptive surfaces of mouse alveolar bone were examined using 3H-thymidine and 3H-proline radioautography. Our results show that the osteoblasts at the appositional surface of the the mouse periodontium are recruited from a progenitor population in the G1 phase of the cell cycle that goes through a single S-phase before differentiating into osteoblasts and that the active lifetime of the osteoblasts on the periodontal surface is approximately 20 days. The lifetime of the osteoblasts on periosteal and endosteal surfaces is 10 days. The active lifetime of an osteoblast appears to be independent of the amount of matrix produced.