The migration of tritiated thymidine ([3H]dThd)-labeled cells toward the epithelial surface was recorded at various intervals (from 1 to 240 hr) in the normal and atypical cervical epithelial of 127 C57BL mice. Cervical atypias were induced by local application of 3,4-benzo[a]pyrene for 5 months9 This treatment was discontinued 7 days before the animals were killed. Cells were labeled by a single ip injection of 6 muCi [3H]dThd/g body weight. The disappearance of heavily labeled cells in normal animals 96 hours after the pulse and in solvent-treated controls after 144 hours suggested that the original labeled cells had been exfoliated. However, cervical atypias showed heavily labeled cells as much as 240 hours after the [3H]dThd incorporation. Universal epithelial labeling (suggesting repopulation by daughter cells) usually occurred 24--48 hours after pulse labeling in normal and control animals but occurred after 96--240 hours in animals with atypical epithelium. On the basis of these results, the pace of both cellular intraepithelium transmigration and of epithelial repopulation by atypical daughter cells is substantially retarded during the early stages of cervical carcinogenesis.