Agmatine, a clonidine displacing substance and imidazoline receptor agonist, was recently isolated from bovine brain and shown to be present in the rat hypothalamus. Since clonidine can stimulate the release of pituitary luteinizing hormone (LH), we tested the hypothesis that agmatine may similarly act in the rat to stimulate the hypothalamic luteinizing hormone-releasing hormone (LHRH)-pituitary LH axis. Administration of agmatine intracerebroventricularly rapidly augmented the release of LH in a dose-related fashion in ovariectomized, ovarian steroid-primed rats. Additionally, agmatine enhanced the in vitro efflux of LH releasing hormone from the median eminence-arcuate nucleus of the hypothalami of rats similarly pretreated with steroids. These studies imply that the endogenous imidazoline receptor agonist, agmatine, may serve as an excitatory neurotransmitter/neuromodulator in the hypothalamic control of LH release and we suggest that the previously reported excitatory effects of clonidine on LH release may be attributed to stimulation by clonidine of imidazoline receptors.