The data from published studies of circadian rhythms in epidermal cell proliferation in mice, rats, and humans were reanalyzed. They were calculated as the percent difference from the mean at six timepoints over 24 h. Composite circadian rhythm curves were plotted from the combined data for each species for S-phase and M-phase. Each group of studies showed a general consensus on timing, and the composite curve showed a regular sinusoidal pattern. The rhythms in mice and rats were the same, whereas those in humans were in the opposite phase and had reduced amplitudes. In the rodents, S-phase peaked at about 3:30 A.M. and M-phase peaked at about 8:30 A.M. In humans S-phase peaked at about 3:30 P.M. and M-phase peaked at about 11:30 P.M. If the timing of the circadian rhythms in cell proliferation can be firmly established, it may be possible to schedule drug treatments to take advantage of the differences in cell proliferation rates at different times of day.