In rats and other mammals, sex differences in the brain and behavior result from differential secretions of gonadal steroid hormones during early critical periods of neural development. We review the experimental results that support current ideas about the mechanisms of sexual differentiation in mammals, and then apply the same experimental analysis to the study of sexual differentiation of the neural song circuit in the zebra finch (Poephila guttata), a passerine song bird. Administration of estrogen to young female zebra finches causes the female to develop a more masculine song system and to sing as an adult. This estrogenic masculinization is similar to that found for copulatory behavior in mammals. However, striking differences emerge in other aspects of the sexual differentiation process. Experiments that use endocrine agents to block the masculine development in genetic males have so far failed. Moreover, the brain of zebra finches has an unusually high expression of aromatase (estrogen synthetase) in the telencephalon, and estrogen synthesized in the brain from androgen is released into the general circulation. These results suggest that the brain is the primary source of estrogen in the body. If so, then a further understanding of sexual differentiation requires more information on the factors that regulate the cerebral synthesis of estrogen.