Five-week-old Brookhaven National Laboratory (BNL), Swiss-albino male mice were maintained on a 12-hour light/dark cycle and were subcutaneously injected with 3H-thymidine (1 micro Ci/gm body weight) one hour prior to sacrifice. Twenty-four mice (three each time-period) were killed at 3 hour intervals for 24 hours. Autoradiographs were prepared from 5-micron thick paraffin embedded decalcified sections of maxillary first molars and surrounding tissues. Labeling index was determined over the cells of the basal layer of the gingival epithelium. Crevicular and crestal gingival epithelium revealed a circadian rhythmic pattern in DNA synthesis in which two peaks were evident. Peak labeling of 5-week-old mouse gingiva exhibited a higher value than previously reported for nontraumatized gingiva. Rodents are nocturnal, as was peak activity, but a second peak is also evident in the light period. Consequently, not all peaks are synonymous with the period of animal activity and feeding. The observed continuous high labeling indices represent a significant potential of gingival epithelium to replenish its cells in response to physiological wear and tear.