Cell migration and the rate at which cells divide was examined in the periodontal ligament of mouse molars. A pulse injection of 3H-Tdr was administered, and mice were sacrificed between 1 hour and 60 days after injection. Analysis of radioautographs using labelling index and grain counts demonstrated that the majority of labelled cells divided within 3 days but a measurable population of cells had not divided after 14 days. These cells were G2-blocked. The labelling index of cells adjacent to alveolar bone increased 8 times within 1 day, indicating that labelled cells had migrated to the bone surface. Migration of cells to the vicinity of the cementum was observed 3 days after labelling. The percentage of labelled cells located within 20μm of one another increased to 40% within 3 days, suggesting clonal proliferation of periodontal ligament cells.