In zebra finches males, but not females, respond to gonadal androgens with the production of a stereotyped courtship song. Corresponding to this dimorphism in hormone sensitivity, two song-related neural regions, magnocellular nucleus of the anterior neostriatum (MAN) and hyperstriatum ventralis pars caudalis (HVc), have more cells that accumulate androgens in males than in females. The early hormonal environment establishes these sex differences in the brain and behavior such that females treated with estradiol (E2) shortly after hatching have male-typical levels of androgen accumulation in MAN and HVc, and respond to adult androgen stimulation with male-typical song. Using autoradiographic procedures employing the androgen 3[H]-dihydrotestosterone (DHT), the present study focused on whether E2 masculinizes androgen target cell numbers in MAN and HVc by inducing, or rather preserving, a male-typical number of androgen target cells. First, it was demonstrated that E2 treatment of females as late as 20 days after hatching increases the number of MAN and HVc cells accumulating 3[H]-DHT or its metabolites in adulthood to levels typical of males. Then the number of androgen target cells present within these nuclei in 20-day-old females was compared with the number present in adult females previously masculinized with E2 treatment on day 20. In MAN, E2 masculinizes androgen accumulation by preserving androgen target cell numbers in the face of massive overall cell loss, a result that suggests that E2 promotes the survival of androgen target cells during sexual differentiation. In HVc, however, E2 treatment on day 20 promotes an addition of androgen target cells, suggesting that during adolescence, E2 regulates the proliferation, migration, and/or survival of these cells.