The composition and proliferation of skin from the ear, tail and footpad oral mucosa form the palate, cheek and tongue of adult mice were examined. The thickness of the nucleated cell layer of skin showed an approximately two-fold variation; a similar range was found between that of oral mucosae but that was considerably thicker than skin. No direct correlation between epithelial thickness and the number of nucleated cells was observed. Proliferative activity, assessed following the administration of vinblastine sulphate, and turnover of the epithelium showed a broad range of activities but more rapid in the oral epithelia than in the epidermis, suggesting a relationship between functional stress and proliferative activity. The criteria used clearly distinguish between morphologically different epithelia and should prove useful examining experimentally produced changes in epithelial histodifferentiation.