Although the experimental bases of contemporary neuroendocrinology were described within the first half of the century, structural sex differences of the mammalian central nervous system were substantiated over the past two decades. Such differences are encountered in brain areas which subserve sex related functions. The structure of these areas appears to be determined by the titers of sex-steroids perinatally, at least in rodents. While important discoveries also suggest a key role of sex steroids in inducing phenotypic and behavioural sex patterns in primates, including humans, this induction does not appear to be solely determined by the steroid environment perinatally. On the other hand, several remarkable structural and histochemical sex differences have been identified in rodents and offer unique models to investigate potential mechanisms by which gonadal hormones bring about neuronal sex differentiation. Thus, some putative mechanisms involved in the inductory role of sex steroids during the development of the nervous system have been proposed. These include the influence of aromatizable gonadal sex steroids during the so-called "critical period" of development. The possible implications of these putative mechanisms in primates remain to be established; however, some human hormonal imbalances during development lead to endocrine and behavioral sex abnormalities, which can be prevented when proper diagnoses and treatment are performed.