Classical theories of sexual differentiation of brain and behavior hold that sex differences in the brain arise because of the action of gonadal steroid hormones. In mammals, testosterone secretion by the testes stimulates a masculine pattern of neural differentiation, whereas feminine patterns of development occur in the absence of testicular secretions. In some bird species, estrogen secreted by the ovary is thought to trigger feminine patterns of neural development, whereas masculine development occurs in the absence of ovaries. Sexual differentiation of the neural circuit for song in zebra finches is not easily explained by these theories. Although female zebra finches can be masculinized by treatments with estrogen, it has proven difficult to prevent masculine neural development in genetic males by treating them with inhibitors of estrogen synthesis. Moreover, when genetic female embryos are treated with inhibitors of estrogen synthesis, they develop significant amounts of testicular tissue that causes little or no masculinization of the song system. Thus, testicular secretions alone appear to be insufficient to cause masculine neural differentiation, and other factors need to be invoked. These factors may include ovarian secretions that inhibit masculine development, or direct genetic (nonhormonal) effects on neural differentiation.