By means of a double‐labelling experiment, circadian variations in the kinetic parameters of the S phase of the hamster cheek pouch epithelium were studied. The evaluation of the experiment included a recently developed correction for deviations from the strict pulse interpretation of the labelling technique. Pronounced circadian variations were found in S phase influx and efflux; the diurnal mean of both was estimated as 0.5%/hr, when based on measurements of all nucleated epithelial cells. Variations in S phase influx seem mainly responsuble for the diurnal variation in cell proliferation, although diurnal variation in DNA synthesis rate, and thus in mean transit time, was also found. The increases in LI and influx were closely correlated and related to the beginning of the dark period. A circadian variation in cell number was also observed.