TitleRapid inhibition of female sexual behavior by gonadotropin-inhibitory hormone (GnIH). Abstract Gonadotropin-releasing hormone (GnRH) is largely responsible for the initiation of sexual behaviors; one form of GnRH activates a physiological cascade causing gonadal growth and gonadal steroid feedback to the brain, and another form is thought to act as a neurotransmitter to enhance sexual receptivity. In contrast to GnRH, gonadotropin-inhibitory hormone (GnIH) inhibits gonadotropin release. The distribution of GnIH in the avian brain suggests that it has not only hypophysiotropic actions but also unknown behavioral actions. GnIH fibers are present in the median eminence (ME) and are in apparent contact with chicken GnRH (cGnRH)-I and -II neurons and fibers. In birds, cGnRH-I regulates pituitary gonadotropin release, whereas cGnRH-II enhances copulation solicitation in estradiol-primed females exposed to male song. In the present study, we determined the effects of GnIH administered centrally to female white-crowned sparrows. A physiological dose of GnIH reduced circulating LH and inhibited copulation solicitation, without affecting locomotor activity. Using rhodaminated GnIH, putative GnIH binding sites were seen in the ME close to GnRH-I fiber terminals and in the midbrain on or close to GnRH-II neurons. These data demonstrate direct effects of GnIH upon reproductive physiology and behavior, possibly via separate actions on two forms of GnRH.