Adult epidermal tissues are renewing cell populations. Mitosis continuously provides new cells replacing those that became keratinized and ultimately shed. It seems plausible to assume that in the epidermis of growing individuals mitosis supplies cells both for cell addition (growth) and renewal. During growth, more cells are presumably retained in the tissue than become lost through desquamation. It is feasible by cutaneous incision to temporarily revert adult epidermis into a growing cell population. Presumably also during healing, cell production exceeds cell loss. What is the magnitude of cell production for renewal of adult epidermis, for simultaneous cell addition and renewal during growth, as well as during epidermal regeneration? To elucidate these problems, the mitotic rates of three epidermal cell populations (ear, plantar, and abdominal epidermis) were determined in the growing, adult, and senile rat. Further, the adult ear and back skin epidermis was converted by cutaneous incision into regenerating cell populations, and the mitotic rates were determined during healing at intervals from six hours to nine days.