It is well established that testosterone from testicular origin plays a critical role in the activation of male sexual behavior in most, if not all, vertebrate species. These effects take place to a large extent in the preoptic area although other brain sites are obviously also implicated. In its target areas, testosterone is actively metabolized either into estrogenic and androgenic steroids that have specific behavioral effects or into inactive metabolites. These transformations either amplify the behavioral activity of testosterone or, alternatively, metabolism to an inactive compound dissipates any biological effect. Androgens and estrogens then bind to nuclear receptors that modulate the transcription of specific genes. This process is controlled by a variety of co-activators and co-repressors that, respectively, enhance or inhibit these transcriptional processes. In addition, recent work has shown that the production of estrogens by brain aromatase can be modulated within minutes by changes in neural activity and that these rapid changes in neuroestrogen production impact sexual behavior, in particular sexual motivation within the same time frame. Estrogens thus affect specific aspects of male sexual behavior in two different time frames via two types of mechanisms that are completely different. Multiple questions remain open concerning the cellular brain mechanisms that mediate testosterone action on male sexual behavior.