Abstract: Birdsong is a species-typical stereotypic vocalization produced in the context of reproduction and aggression. Among temperate-zone songbirds, it is produced primarily by males, and its frequency and quality are enhanced by the presence of the gonadal steroid hormone testosterone in the plasma. In the brain, the effects of testosterone on song behavior involve both estrogenic and androgenic metabolites of testosterone that are locally produced and act via their cognate receptors. Androgen, and in some cases estrogen, receptors are present in many specialized forebrain song control nuclei. Testosterone can regulate catecholamine steady-state levels and turnover in these song control regions. Tracing studies combined with immunocytochemistry for tyrosine hydroxylase (a marker of catecholamine synthesis) reveal several catecholamine cell groups that project to forebrain song control nuclei. These brain areas also express the mRNA for either androgen receptors or estrogen receptor alpha, and androgens enhance the expression of tyrosine hydroxylase. Dopaminergic cell groups that project to song nuclei express the protein product of the immediate early gene fos in association with the production of territorial song. Thus, testosterone may be acting on song behavior via these ascending catecholamine cell groups. Chemical lesioning studies suggest that noradrenergic projections to the song system are involved in the latency to produce song and the ability to discriminate conspecific from heterospecific song. The song control circuit may thus be modulated in significant ways via the androgen regulation of forebrain catecholamine systems.