[en] Binding sites for rat atrial natriuretic eptide (6-33) (ANP) were quantitated in the subfornical organ of chronically dehydrated homozygous Brattleboro rats unable to synthesize vasopressin; heterozygous Brattleboro rats, their controls, Long Evans rats and Long Evans rats after 4 days of water deprivation. Brain sections were incubated in the presence of ¹²⁵I-ANP and the results analyzed by autoradiography coupled to computerized microdensitometry and comparison to ¹²⁵I-standards. Brattleboro rats and water deprived Long Evans rats presented a higher number of ANP binding sites than their normally hydrated controls. The results suggest a role of ANP binding sites in the subfornical organ in the central regulation of fluid balance and vasopressin secretion