A trial natriuretic peptide (ANP) is activated during ECF volume expansion, however, the role of ANP during ECF volume contraction is not clearly understood. We undertook the present study to investigate whether ECF volume contraction induced by sodium or chloride depletion modulated ANP binding sites in two brain areas involved in the regulation of water intake and vasopressin release, the subfornical organ (SFO), located outside the blood brain barrier, and the paraventricular nuclei (PVN), located inside the blood brain barrier. Three groups of eight Male Sprague-Dawley rats 45 ± 5 g body weight, were fed either control (0.3% NaCl), Na+ deficient(Na+ 0.005%), or Cl- deficient (Cl- 0.005%) diets for 35 days. ANP plasma levels were significantly decreased in both Na+ and Cl- depleted groups. Cl- depleted rats developed a more significant reduction in ECF volume than sodium depleted rats, since AVP serum levels increased despite a low serum osmolarity. By ex-vivo quantitative autoradiography we found that SFO ANP receptors linked to guanylate cyclase(ANP A and ANP B), were significantly downregulated (30%) by sodium depletion, but not significantly affected by Cl- depletion. No significant changes were present in ANP receptors located in the PVN nuclei, suggesting that these receptors may be under the influence of other stimuli. To determine whether ECF volume contraction per se affected SFO receptor number, an additional group of young rats was subjected to water deprivation. Water deprivation upregulated ANP receptors in the SFO and increased AVP mRNA expression in specific brain areas, suggesting that ANP SFO receptors may be involved in the regulation of vasopressin release. Thus, ANP SFO binding sites may represent physiological active receptors sensitive to changes in ECF volume-electrolyte balance with the ability to link stimuli from the central and peripheral ANP systems and to modulate vasopressin release in ECF volume contracted young rats.