The nuclear receptor steroidogenic factor 1 (SF-1) regulates the biosynthesis of the two essential mediators of male sexual differentiation, androgens and Müllerian-inhibiting substance, and is required for adrenal and gonadal development and gonadotropin expression. SF-1 is also expressed in the embryonic ventral diencephalon, subsequently localizing to the ventromedial hypothalamic nucleus, a region important for reproductive behavior. Mice lacking SF-1 secondary to targeted disruption of the Ftz-F1 gene had normal numbers and location of GnRH neurons but exhibited grossly impaired ventromedial hypothalamic nucleus structure. Despite their apparently normal GnRH neurons, treatment of Ftz-F1-disrupted mice with GnRH restored pituitary gonadotropin expression. These studies define SF-1's essential role within a discrete hypothalamic nucleus previously linked to reproduction.