Atrial natriuretic peptide (ANP) binding sites were measured by quantitative autoradiography in the supraoptic and paraventricular nuclei and in the subfornical organ of hypophysectomized, adrenalectomized, and genetically vasopressin-deficient (Brattleboro) rats. Hypophysectomized and Brattleboro rats had significantly higher numbers of ANP binding sites in the supraoptic nucleus and in the magnocellular subdivision of the paraventricular nucleus than their respective controls. ANP binding density was also increased in the parvocellular subdivision of the paraventricular nucleus in hypophysectomized rats and in the subfornical organ of homozygous Brattleboro rats. When homozygous Brattleboro rats were treated with vasopressin, the density of ANP binding sites was restored to control level in the subfornical organ but not in the supraoptic or paraventricular nuclei. Adrenalectomy did not influence ANP binding in the brain areas studied. Increased ANP binding density in Brattleboro rats and after hypophysectomy in the nuclei in which vasopressin neurons are located suggest that ANP binding sites may represent physiologically active receptors and may mediate the inhibitory action of ANP on vasopressin secretion.