In zebra finches, androgens stimulate the production of a learned courtship song in males but not in females. Corresponding to this behavioral dimorphism, neural regions controlling the learning and production of song are much larger in males than in females. In two of these song-related brain regions, magnocellular nucleus of the anterior neostriatum (MAN) and hyperstriatum ventrale pars caudale (HVc), males have a larger percentage of androgen-accumulating cells than females. Since sex differences in the capacity for song and in the size of songrelated nuclei are established by gonadal hormones shortly after hatching, we determined whether the early hormonal environment also establishes sex differences in androgen accumulation within MAN and HVc. Newly hatched female zebra finches received either estradiol (E2) or cholesterol (Ch). Three to six months later, E2-females, Ch-females, and normal adult males were gonadectomized and injected 24 hr later with 3H-dihydrotestosterone. Autoradiograms were prepared, and the incidence of androgen-labeled cells was determined for MAN, HVc, and a control region, the lateral septal nucleus (SL). In females, early E2 exposure dramatically increases the percentage of androgen-accumulating cells in MAN and HVc, without influencing androgen accumulation in SL. In MAN and HVc, the percentage of androgen-concentrating cells in E2-females approximates that observed in normal adult males. Cells also tended to be more densely labeled in E2-females than in Ch-females. Since early E2 exposure renders the female song system neuroanatomically and functionally responsive to androgens, we suggest that E2 establishes this responsiveness by regulating the number of androgen target neurons within MAN and HVc.