Samples of epidermis, and in some cases tongue epithelium, were taken from tritiated thymidine labelled mice at three hourly intervals throughout the day, daily intervals for a period of two weeks and weekly intervals for a period of 15 months. Samples were counted for total radioactivity in a liquid scintilation counter while other samples were prepared for autoradiography and the determination of the labelling index (LI) (number of cells replicating their DNA). Clear daily (circadian rhythms) were evident. There was no sign of any rhythm within the period of a week or within a month. The radioactivity levels in fresh and fixed skin showed a 16 week periodicity which was clear from a Fourier transform of the data. There was considerable scatter amongst the data and the LI data for epidermis and radioactivity measurements for tongue epithelium both showed a dramatic change in the Spring after which the average values were elevated and more scatter was apparent. It is not clear whether this is a reproducible seasonal effect.