In the brain, conversion of androgens into estrogens by the enzyme aromatase (estrogen synthase) is a key mechanism by which testosterone regulates many physiological and behavioral processes, including the activation of male sexual behavior, brain sexual differentiation and negative feedback effects of steroid hormones on gonadotropin secretion. Studies on the distribution and regulation of brain aromatase have led to a new perspective on the control and function of this enzyme. A growing body of evidence indicates that the estrogen regulation of aromatase is, at least in part, trans-synaptic. Afferent catecholamine pathways appear to regulate aromatase activity in some brain areas and thereby provide a way for environmental cues to modulate this enzyme. The localization of aromatase in pre-synaptic boutons suggests possible roles for estrogens at the synapse.