Atrial natriuretic peptide [rat (r) ANP6-33 or ANP99-126] binding sites were localized in discrete areas of rat brain and pituitary gland using quantitative autoradiographic techniques. High numbers of rANP6-33 binding sites were concentrated in the circumventricular organs (the organon vasculosum laminae terminalis, organon subfornicalis, and area postrema) and selected hypothalamic nuclei (the nucleus supraopticus, nucleus preopticus medianus and nucleus paraventricularis). High binding was also present in the choroid plexus and the bulbi olfactorii (laminae medullaris interna). A relatively low number of rANP6-33 binding sites was observed in other olfactory, limbic and brainstem areas (the nucleus tractus solitarii, nucleus motoris dorsalis vagii and nucleus hypoglossi), the eminentia mediana and the pituitary gland (anterior and posterior lobes). High-affinity rANP6-33 binding sites were demonstrated in the organon subfornicalis and the area postrema after incubation of consecutive sections from individual rat brains with 125I-rANP6-33 in concentrations from 20 to 400 pM. rANP6-33 binding sites were concentrated in areas associated with angiotensin II and/or vasopressin, suggesting an interaction among these peptides in the central nervous system.