In order to know whether gonadotropin-releasing hormone (GnRH)-like substances play a role as local regulators in the ovary, localization of the GnRH receptor, and the effects of GnRH agonist (GnRHa) on the cyclic AMP system in the rat ovary was studied. In addition, effects of GnRH antagonist on the rat ovary and effects of GnRHa on steroidogenesis in the human ovary were also examined. In the rat, localization of the GnRH receptor was observed to change with follicular development, and GnRHa decreased follicle-stimulating-hormone-stimulated cAMP production. On the other hand, GnRH antagonist showed the tendency to promote ovarian function. In the human ovary, GnRHa seemed to inhibit steroidogenesis directly, in vivo and in vitro. These data suggested that GnRH/GnRH-like substances play a role as local regulators in the ovary.