Abstract: Bain regions controlling courtship song in the zebra finch are larger in male than female. We found robust expression of androgen receptor (AR) mRNA in Area X as well as in other song regions. Estrogen treatment of females increased AR mRNA expression in Area X, whereas treatment of males with the aromatase inhibitor, fadrozole, decreased AR expression in Area X at P11. Thus estrogen is necessary for full masculine AR expression in the song system, and the estrogenic up-regulation of AR contributes to subsequent differential actions of androgen in song nuclei. Using retinoic acid synthesizing enzyme (zRalDH) mRNA as a region-specific marker to identify song regions, we found zRalDH expression in HVC and RA as early as P4-5, but no sex difference in the volume or intensity of zRalDH and AR expression in HVC and RA at P11. The zRalDH defined volume of HVC in females at P11 was significantly greater than that in adult females, whereas the volume of HVC in adult males was substantially larger than that in males at P11. These results suggest that HVC in females experience net cell loss and HVC in males might recruit more cells between P11 and adulthood. We found the expression of NGF and its high affinity receptor, trkA, in restricted regions of the telencephalon, usually in the same regions, suggesting that NGF acts on trkA close to its synthesized sites. NGF and trkA were detected in adult HVC, RA, and lMAN. At P25 the intensity and volume of RA defined by NGF and trkA in males was slightly higher than that in females, suggesting that NGF and trkA might play a role in differential cell survival in lMAN and RA to produce sex differences. Using doublecortin (DCX) as a marker for migratory neurons, we found the expression intensity at P9 was substantially higher than that of adults, indicating that DCX expression was developmentally regulated. No sex difference was found in DCX expression at P9 or in adult except that labeled Area X in adult males. If DCX is predominantly expressed in migrating neurons, as reported in mammals, these results offer no evidence for sex differences in neuronal migration.