The distribution of gonadotropin-releasing hormone (GnRH) was studied immunohistochemically in the brain of the adult mouse with the peroxidase-antiperoxidase (PAP) method. Primary antisera were prepared against unconjugated synthetic GnRH and GnRH conjugated to limpet hemocyanin or bovine serum albumin. GnRH was localized in the organum vasculosum of the lamina terminalis (OVLT), with the greatest amount being found ventral to the cephalic end of the third ventricle. In the cephalic region of the median eminence, it was concentrated bilaterally in longitudinal bands located dorsal to the tuberoinfundibular sulci. More caudally, near the junction of the infundibulum with the brain, GnRH accumulated over the apex of the tuberoinfundibular sulci, with several foci being scattered from this region medially to the ependyma of the third ventricle. The greatest aggregation of GnRH occurred in the psotinfundibular median eminence in an area extending from the floor of the third ventricle to the ventral surface of the brain. In the caudal median eminence, GnRH was arranged in a narrow ventral band that crossed the midline. GnRH appeared to be located in axonal processes and terminals of the OVLT and median eminence; the structures observed were granular. GnRH was not localized within the neuronal cell bodies of any hypothalamic nuclei. When one antiserum to conjugated GnRH was used at high concentration, the cytoplasm of ependymal cell bodies and tanycyte processes was stained more intensely than the general background, but, with absorption and/or dilution of the antiseru, this staining was shown to be nonspecific.