The carcinogen 4NQO was applied to the palates of rats for 2 weeks or 2 months. Eight weeks after termination of carcinogen application, animals were injected with colchicine and sacrificed at intervals of 3 h during a 24 h period. Histological sections were produced and the number of arrested metaphases counted in the palatal epithelium. The two groups had the same daily mitotic rate and exhibited a circadian rhythm in mitotic activity similar to that demonstrated in normal oral mucosa of nocturnal animals. Carcinogen was applied to the palates of other groups of rats for 2 weeks or 2 months. Animals treated with saline served as controls. At various time points during and after carcinogen treatment eight rats from each group were killed. Four of these animals were injected with colchicine prior to sacrifice and the number of arrested metaphases counted in the palatal epithelium. In the noncolchicinized rats all mitotic phases were counted. After cessation of carcinogen application the mitotic rates, the mitotic counts and the mitotic durations did not differ from those of control animals, indicating that the carcinogen had failed to induce irreversible changes in the cell population kinetics of the palatal epithelium.