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Immediately after plucking the hairs from a mouse, with its skin in the resting phase of the hair growth cycle, both the labeling index and incorporation levels of ³HTdR are lowered. This depression persists until stimulated cells enter S, usually about 10 hours after plucking. These observations are confirmed using a combination of liquid scintillation and autoradiographic procedures. The dpm per punch of fixed skin, the labeling index of the basal layer, the grains per labeled basal layer cell and the proportion of labeled DNA are all reduced following plucking. The dpm per punch of fresh skin is concomitantly enlarged by plucking through an increase in the fixative-soluble pool, which may be due to the induction of vascular changes. Anesthesia apparently reduces slightly the incorporation levels into fixed unplucked skin. The vascular changes and anesthesia depression of incorporation cannot account for the depression in incorporation into fixed tissue which must be due to a more direct effect on DNA synthesis. The data could be explained on the basis of a reduction in the rate of synthesis combined with a block to entry into S or to progression through S. Using the autoradiographic and liquid scintillation techniques presented in this paper and published previously, a number of calculations were made regarding the proliferative population size in the epidermis and hair follicle. The cycle time for basal layer cells in unplucked skin is estimated to be 141 hours with an upper limit on the growth fraction of 0.92. [ABSTRACT FROM AUTHOR]