Binding sites for rat atrial natriuretic peptide (6-33) (ANP) were quantitated in the subfornical organ of chronically dehydrated homozygous Brattleboro rats unable to synthesize vasopressin; heterozygous Brattleboro rats, their contro'ls, Long tvans rats and Long Evans rats after 4 days of lrater,qejrivation. Brain sections were incubated in the presence of r¿)l-ANP and the results analyzed by autoradiogr?ehy coupled to computerized microdensitometry and coflparison to IZ5l-standards. Brattleboro rats and water deprived Long Evans rats presented a higher number of ANP binding sites than their noñnally hydrated controls. our results suggest a role of ANP binding sites in the subfornical organ in the central requlation of fluid balance and vasopressin secretion.