Topical application to the guinea pig skin of either ethanol, isopropyl palmitate and n-hexadecane in combination or n-hexadecane and ethanol separately produces characteristic circumscribed hyperplasia of the epidermis. The degree and intraepidermal distribution of these changes can be quantitatively assessed by epidermometric methods. Simultaneous application of topical corticoids leads to an inhibition of epithelial proliferation, the degree of which can likewise be determined epidermometrically. Under appropriate experimental conditions, epidermoplasia inhibition tests furnish quantitative evidence of the antihyperplastic activity of locally applied corticosteroids on the epidermis. This aspect of their overall therapeutic action cannot be evaluated by other test methods.