The pattern of epidermal cell proliferation, following a single application of a hyperplasia inducing agent, can well be interpreted in cybernetic terms. This concept implies that the local concentration of a possible growth inhibiting signal substance should exhibit variations compatible with the changes seen in the cell kinetics of the epidermis. This signal substance has been called the epidermal chalone, first preliminary characterized by Bullough et al. (1964c). Meanwhile a considerable number of other chalone systems have been identified in different organs. From the experimental results a chalone was considered an anti-mitotic substance, which is synthesized within the same tissue on which it specifically acts. It was found not to be species-specific; it reveals a rapid and reversible inhibition by diffusion and passes throughout the tissue and into the blood. Later investigations have exhibited that the epidermal chalone probably consists of at least two separate compounds (proteins), one acting on cells in the postsynthetic, pre-mitotic G 2-phase, the other one on cells in late G 1 (Marks, 1971; Elgjo et al., 1971, 1972). The proliferative behavior of epidermal cells following an application of different irritants was the main subject of the present investigations. All experiments were performed in vivo with hairless mouse epidermis, which was treated with 20-methylcholanthrene (MCA) as carcinogen and crotonoil as cocarcinogen, both dissolved in acetone, and with repeated Scotch tape stripping. Different methods were employed in order to determine epidermal proliferation parameters such as: a. the cytophotometrically measured amount of nucleic deoxyribonucleic acid (DNA) of Feulgen-stained epidermal basal cells, b. the total DNA content of whole epidermis, c. the number of Colcemid(R) arrested metaphases (mitotic rate) in the basal cell layer, d. the incorporation of H3-thymidine into DNA of epidermal basal cells. The number of epidermal basal cells revealing an increased DNA content is significantly lowered for more than 24 hours after MCA and for 12 hours after Crotonoil administration. This effect was not observed after Scotch tape stripping. In all the treated groups this period was followed by a proliferation wave exhibiting more cells with an increased DNA-content during the first 6-10 days after irritation. Only after MCA treatment this higher cell number appeared with a lag phase of 3 to 4 days and coincided with a decrease of the total amount of epidermal DNA. All irritants produced a marked epidermal hyperplasia during the first two weeks after application.