Receptors for testosterone (T) in the preopticoanterior hypothalamus (PO) and in the arcuate nucleus and median eminence (ARC + ME) were examined in neonatally androgenized female and neonatally castrated male rats. As a result of neonatal castration of males, the concentration of cytoplasmic and nuclear receptors for T in the PO dropped to an undetectable level. In the ARC + ME, the number of T-binding sites in the cytosole fraction remained unchanged, while that in the nuclear fraction decreased 2-fold. In the cytosole fraction of neonatally androgenized females, receptors for T were detectable only in the ARC + ME, the level of binding being not different from that seen in this hypothalamic area in intact and neonatally castrated males. At the same time in the nuclear fraction, receptors for T were detectable in both hypothalamic areas, the number of T-binding sites in the ARC + ME being 1.5 times less than in the PO. The data obtained attest to the involvement of receptors for T in sexual differentiation of the brain and regulation of gonadotropic function of the hypophysis.