The diurnal cycles of mitotic activity in the ear epidermis of the adult male mouse have been determined by the removal of earclips at 2 hr. intervals throughout the 24 hr. The mice used were between 3 and 4 months old, and were of the Kreyberg white label and Strong's CBA strains. A considerable degree of individual variation was found, but on the average the maximum mitotic activity was at 06.00 and 14.00 hr. and the minimum mitotic activity at 10.00 and 20.00 hr. This observation was confirmed by killing groups of mice, each group consisting of five males, at the same 2 hr. intervals throughout the 24 hr. Similar variations in the mitotic activity of the ear epidermis were observed, and, in addition, similar cycles were evident in the mid-dorsal epidermis of the back, the stratified epithelium of the oesophagus, the lining epithelium of the epididymis, and the proliferating zone of the duodenal mucosa. In this last tissue the rate of cell division never fell to a very low figure, and in the proliferating centres of the intestinal lymph nodules and in the seminiferous tubules of the testis there was no trace of a cycle since the rate of cell division remained constantly high. A study was also made of the spontaneous bodily activity of the mice throughout the 24 hr., and by comparing the average figures so obtained with the average figures for epidermal mitosis, it proved possible to make the significant correlation that when the animals are at rest mitotic activity is at a maximum and that when they are awake and active it is at a minimum. This correlation permits an explanation of the individual variation in mitotic activity, since there is also a high degree of individual variation in spontaneous bodily activity. It also permits an explanation of the contradictory results which have been reported in the past regarding diurnal mitosis rhythms in mice, since it is evident that the rhythms of bodily activity must be strongly affected by differences in the age, sex and condition of the animals used, in the season of the year, and in the routine of the laboratory.