Cytogenesis of the developing molar periodontal ligament was studied by determining labelling indices in 4 groups of young mice. Autoradiographs were made using paraffin sections of demineralised specimens from 10 days, 12 days, 16 days and 20 days old mice allowed to survive a varying period (1--96 hours) after the administration of [3H]thymidine. Labelled fibroblasts in number per unit area were counted over three different zones (apical, middle and cervical) of the sections and labelling index distribution curves were drawn. The present study has demonstrated the existence of a progenitor area with proliferating fibroblasts in the apical zone and a migration of these cells into other zones (middle and cervical). However, it has also shown that a measurable proliferative rate is present at all levels of the molar periodontal ligament in all ages of animals investigated. Such an observation if compatible with fibroblasts migrating from the apical zone in an occlusal direction and maintaining a decreasing degree of proliferative activity as they progress. Consequently, a tentative apico-occlusal migratory model for the periodontal ligament fibroblast proliferation and migration is proposed. Accordingly, fibroblast proliferation takes place in the apical progenitor zone, and cells migrate from the apical zone to middle and cervical zones, but maintaining a decreasing degree of proliferative activity as they migrate. Peak proliferative and migratory activity is seen in the 12 days group which is also the time that tooth eruption takes place in the mouse. Thus, a close relationship exists between fibroblast cell proliferation, migration and tooth eruption.