Extracellular norepinephrine (NE) levels in the paraventricular/anterior hypothalamic area (P/A) and in the dorsomedial medulla (DM) in conscious Sprague-Dawley rats were estimated by in vivo microdialysis before, during, and after sustained hypotension (75 mmHg mean arterial pressure) produced either by hemorrhage (Hem) or by 2-chloroadenosine infusion (2-Cl-ADO, 2.6-26.0 micrograms/min iv). P/A and DM NE were also measured before, during, and after hypertonic saline infusion (HTS; 1.5 M NaCl at 10 microliters.100 g-1.min-1 iv). P/A and DM NE increased during both Hem and 2-Cl-ADO and returned to baseline after reinfusion of hemorrhaged blood or after 2-Cl-ADO was stopped. However, Hem caused greater increases in P/A NE than 2-Cl-ADO despite equivalent decreases in blood pressure. Hem and 2-Cl-ADO produced equivalent changes in DM NE. HTS did not change P/A or DM NE despite increases in blood pressure of approximately 15 mmHg and plasma osmolality of approximately 30 mosmol/kgH2O. We conclude that 1) hypotension increases P/A and DM NE, which may mediate compensatory responses, 2) Hem is a more potent stimulus for NE release in the P/A than isovolemic hypotension induced by 2-Cl-ADO, and 3) the hypertensive response to HTS does not involve changes in P/A or DM NE.