The neuropeptide gonadotropin-releasing hormone (GnRH, LHRH) serves as both a hormone and a neurotransmitter, and it has multiple actions on reproductive physiology and behavior. At least seven different molecular forms of GnRH have evolved, and nearly all vertebrates studied express at least two different forms of GnRH: chicken GnRH II, and a second form that varies across classes. The GnRH cell bodies span a broad region of the forebrain and midbrain, and processes project to virtually every region of the CNS, to the vasculature, and to the cerebrospinal fluid. Comparative evidence supports the model that gnathostomic vertebrates possess two principle GnRH systems, with different embryonic and, probably, evolutionary origins, expressing different molecular forms of GnRH and projecting to different targets. The terminal nerve-septo-preoptic system serves as the principle regulator of gonadotropin release in most vertebrates. Neurons originate in the embryonic olfactory placode and migrate centrally during development, and it is proposed that ontogeny of the TN-septo-preoptic system reflects its evolutionary origins as a peripheral endocrine organ associated with the olfactory system. The second GnRH system, which arises from non-placodal precursors, comprises cell bodies in periventricular regions of the posterior diencephalon and/or midbrain. Although much less is known about the posterior GnRH system, evidence suggests that these cells served as the ancestral brain GnRH system and are the cellular locus of the chicken GnRH peptide. The GnRH system of lampreys does not appear to be homologous to either the TN-septo-preoptic or posterior GnRH system, and it is suggested that GnRH in agnathans represents a third evolutionary event.