Five men with healthy skin ranging in age from 24 to 37 years were investigated for the presence of diurnal variation in epidermal cell proliferation by the in vitro 3H-thymidine labeling technique. The following parameters were studied: The basal cell LI; the numbers of labeled basas and supra-basal cell nuclei as well as the total number of labeled nuclei situated above a basal membrane length of 100 mum. All autoradiographic parameters showed marked diurnal variations, the amplitudes of which differed individually. The basal 3H labeling index always showed the highest diurnal variation. In all patients diurnal variations of the basal and suprabasal cell proliferation occurred simultaneously and in equal direction. However, each test person showed maxima and minima of epidermal DNA synthesis at different times. No synchronism could be found when comparing the individual variations of cell proliferation. Several possible reasons for the asynchronous in vitro behaviour of epidermal cells will be discussed.