Without question, intracellular metabolism has a profound influence on the mechanism of action of androgens, adding an aspect of complexity and biological subtlety not shared by other classes of steroid hormones. In the final analysis, the metabolic potential and capability of an androgen target cell will be of equal im portance to the presence of androgen receptor systems in the overall expression of biological responses. Furthermore, it should always be borne firmly in mind that the activity of many enzymes is subject to modulating influences of development, differentiation, and the hormonal milieu. This is of critical importance in the planning and subsequent interpretation of experiments on androgenic regulation. It would also be narve to assume that all androgenic responses are mediated by the intracellular formation and binding of 5α-dihydrotestosterone; it is now abundantly clear that certain biological phenomena are specifically regulated by testosterone itself, other active 5α-reduced metabolites, 5β-reduced metabolites, and even oestrogens.