Subcutaneous injection of epidermal growth factor (EGF) into the pinna of adult female and castrated male Syrian hamsters resulted in an increase in the number of cells per sebaceous gland unit. The effect of EGF on the sebaceous cell number was localized to the treated ear and accompanied by epidermal hyperplasia. The injection of testosterone into the ear also produced an increase in the cell number. When testosterone and EGF were injected together, the two hormones exerted an additive effect on the sebaceous glands of female hamsters. This is the first demonstration of a sebotrophic action of this polypeptide hormone.