The frequency of labeled mitoses method (FLM) was used to study the cell kinetics in the stratified squamous epithelium of the mouse esophagus and tongue. FLMs were generated by injecting tritiated thymidine (TdR) at two different phases of the mouse circadian system: TdR was injected into one group of mice at 0900 and into a different, second group of mice at 2100. Three variables were monitored for each group; (1) the FLM, (2) the mitotic index and (3) the grain count over the labeled mitotic figures. In both the esophagus and the tongue there was a circadian rhythm in the mitotic index with the peak occurring during the first half of the diurnal phase and the trough occurring during the first half of the nocturnal phase. The FLM curves from each group revealed the following data.