Purpose: Hyponatremia is the most common electrolyte disorder and in 2006 the cost of treating hyponatremia in the US was estimated to be $1.6-$3.6 billion per year. Inappropriate vasopressin secretion is the major cause of dilutional hyponatremia associated with liver and heart failure. Our previous studies have also shown that, in male BDL rats, BDNF-TrkB signaling in the SON contributes sustained AVP and copeptin (a clinical surrogate for AVP 1-3) release, and hyponatremia 4. However, little is known about the role of sex differences in hyponatremia5. Our data show that female BDL rats do not develop hyponatremia or increased AVP release6. Instead female BDL rats show increased release of oxytocin6 . Our findings also suggest that the effects of BDL on neurohypophyseal hormones may be sexually dimorphic. Our central hypothesis is that ER in the hypothalamo-neurohypophyseal system (HNS) contribute, at least in part, to sex differences seen in the BDL model of dilutional hyponatremia. Our hypotheses will be tested with the following Specific Aims: Aim 1 will define the contributions of estrogen receptors in hypothalamo-neurohypophyseal neurons to sex differences in a model of dilutional hyponatremia Aim 2 will determine differences in SON gene expression that may contribute to underlying sex differences in AVP release in a model of dilutional hyponatremia Benefit: These experiments will address an existing gap in our understanding of neurophypophyseal function and the pathogenesis of hyponatremia. The findings of these experiments could potentially alter the way that inappropriate vasopressin release is studied and conceptualized clinically.