Using tritiated vasopressin, specific binding sites for vasopressin were identified on splenic membranes of Sprague-Dawley rats. One class of high affinity receptors was characterized with an equilibrium dissociation constant of 1.91 +/- 0.16 nM and a maximal binding capacity of 110 +/- 11 fmol/mg of protein. Several experimental evidences suggest that these receptors belong to the V1-vascular type: The affinity of 8 vasopressin agonists for the receptor is correlated to their vasopressor activity in vivo whereas no such relationship exists when their antidiuretic activity is considered. The affinity of 5 vasopressin antagonists for the receptor is correlated to their antivasopressor activity in vivo whereas no such relationship exists when their antidiuretic activity is considered. Vasopressin does not stimulate cyclic AMP production of splenic membranes. The regulation of these receptors was studied in sodium-sensitive (S) and sodium-resistant animals (R) of Dahl receiving a special diet (0.1 or 8% NaCl) for two weeks. In both strains of animals, the number of receptors is smaller in R animals than in S animals, whatever the diet ingested. Splenic membranes of rat bear easily accessible V1-vasopressinergic receptors. These receptors are modulated by sodium intake and the difference in receptor number between S and R rats could explain the increased vascular reactivity of S animals to vasopressin.