Central nervous system derived adrenomedullin (AM) has been shown to be a physiologic regulator of thirst. Administration of AM into the lateral ventricle of the brain attenuated water intake, while decreased endogenous AM, by an AM-specific ribozyme, led to exaggerated water intake. We hypothesized that central AM may control fluid homeostasis in part by regulating plasma arginine vasopressin (AVP) levels. To test this hypothesis AM or a ribozyme specific to AM was administered i.c.v. and alterations in plasma AVP concentrations were examined under both basal and stimulated (hypovolemic) conditions. Additionally, we examined changes in blood volume, kidney function, plasma electrolyte and protein levels, as well as changes in plasma aldosterone concentrations. Intracerebroventricular administration of AM increased plasma AVP levels while AM-ribozyme treatment led to decreased plasma AVP levels under stimulated conditions. During hypovolemic challenges, AM-ribozyme treatment led to an increased loss of plasma volume compared to control animals. While overall plasma osmolality did not differ between treatment groups during hypovolemia, AM-ribozyme treated rats had significantly higher aldosterone levels and consequently lower plasma potassium concentrations compared to controls. These data suggest that brain-derived adrenomedullin is a physiologic regulator of vasopressin secretion and thereby fluid homeostasis.