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Summary In human beings, androgen metabolism plays an important role in mediating the action of male hormones upon target structures of the skin. First, human skin is capable of transforming inactive steroids supplied through the blood, such as androstenedione and dehydroisoandrosterone, into the active androgen testosterone. Second, human skin is able to reduce testosterone to 5alpha-DHT, an essential prerequisite, during embryogenesis, for the male differentiation of target structures derived from urogenital sinus. At puberty, hair growth in sexual areas of skin also requires the transformation of testosterone to DHT. Regulation of 5alpha-reductase activity varies according to the anatomical site of the enzyme. In foetuses, 5alpha-reductase activity present in tissues derived from the urogenital tract does not seem to be androgen-dependent, since it is acquired before the onset of testosterone secretion by foetal testis. By contrast, the enzyme that mediates development of certain secondary sex characteristics, such as pilosebaceous gland activity in sexual areas, is clearly androgen-dependent, since it is absent before puberty and in persons with hypogonadism. These differences in the control of the 5alpha-reductase activity mediating the appearance of either primary or secondary sex characteristics are important and may explain the differences in 5a-reductase activity observed in adult skin of both sexes derived from different sexual areas. In addition, the knowledge of androgen relation to the skin is necessary in order to understand the action of the anti-androgens, particularly the compounds which may be used by topical administration.