Androgens mediate a wide range of processes during embryogenesis and in the adult. In mammals, the principal androgens are testosterone and its 5alpha-reduced metabolite, 5alpha-dihydrotestosterone (DHT). Although these androgenic hormones are diverse in character, it is believed that their effects are mediated via the protein products of a single androgen receptor gene encoded on the X-chromosome. A great deal of information has now accumulated pertaining to the mechanisms by which nuclear receptors, such as the androgen receptor, modulate the activity of responsive genes. The studies have demonstrated the participation of a number of ancillary proteins in modulating activation or repression by nuclear receptors. In addition to studies focused on the mechanisms of nuclear receptor function, additional work has illuminated the mechanism by which androgens are metabolized in selected tissues. This information provides a perspective on the number of levels of complexity by which differential gene regulation by androgens may occur in different tissues and in different cell types.