Pulse labeling of spontaneous, stimulated or disturbed epidermal cell reinforcements with tritium thymidine offers a simple system to appraise relations of different phases in cell cycle. For example, if the duplication of DNA during the S-phase takes much time, many cells actually synthesizing DNA must be found labeled. Therefore, a high labeling index does not necessarily indicate fast cell cycles (and fast cell cycles do not necessarily produce high labeling indices). The slower S-phases proceed, the less use is for tritium thymidine, offered during the short availability time of a single pulse label. As a result, only few silver grains per cell nucleus are produced in the film emulsion by autoradiography. The typical pattern of this DNA-synthesis would be a high labeling index, combined with a low silver grain index. All other possible combinations and degrees of high, low, normal silver grain indices with high, low or normal labeling indices allow to appraise not only the S-phase duration in comparison with controls, but also in dependence of the other phases, mainly G1, and their duration in cell cycle.