An increase in plasma osmolality induced by dehydration causes an increase in sympathetic nerve activity (SNA). The central nervous mechanisms underlying the increase in SNA after dehydration are not fully established. Here, we investigated the sequential effects of systemic administration of Losartan (20 μM, angiotensin II type 1 (AT1) receptor antagonist), brain transections and chemical inhibition of commissural nucleus tractus solitarii (cNTS) on ongoing thoracic SNA after chronic dehydration (DH; 3 days of water deprivation). Subsequent to DH, experiments were performed in the in situ working heart-brainstem preparation of rat (Antunes et al, 2006). Under deep halothane anaesthesia (assessed by an absence of a limb withdrawal reflex to noxious pinching), the rat was transected below the diaphragm, decorticated to make insentient and perfused with oxygenated Ringer’s solution via the descending aorta. Perfusion pressure, heart rate, phrenic nerve activity and thoracic SNA were recorded. Data are expressed as mean ± SEM. In euhydrated (EH) rats (290 mOsmol perfusate), systemic application of Losartan and subsequent pre-collicular transection (to remove the hypothalamus) reduced SNA by -20±3% and -44±2% respectively (n=5;P<0.05). In contrast, in DH rats (340 mOsmol perfusate,n=6) Losartan, subsequent pre-collicular and then pontine transections failed to reduce SNA (-1±4%, -3±8% and -12±4%; P=0.753, P=0.995 and P=0.372, ANOVA). However, transection at the medulla-spinal cord junction reduced SNA by -70±8%. In intact DH, but not EH rats, reversible inactivation of cNTS using isoguvacine, (a GABAA receptor agonist; 100 mM, 100 nl), reduced significantly baseline SNA (-33±7% in DH; P<0.01 vs 3±8% in EH). Since it was demonstrated that dehydration increases FosB staining in the cNTS (Ji et al, 2007), we chronically blocked AP1 transcription factor activity in the cNTS using a viral vector expressing a FosB dominant negative (Ad-CMV-IRESEGFP-dnFosB; 2.9×10⁶ pfu/μl) 5-7 days prior to 3 day water deprivation. In these animals, inactivation of the cNTS was ineffective (-6.9±8.7% vs. -49±8%, control rats with Ad-CMV-eGFP in NTS; P < 0.05, Student’s t test). In addition, in DH rats in which AP1 was blocked in the cNTS, SNA was now decreased after pre-collicular transection (-45±8%), a response that was similar to that seen in control EH rats (-35±6%). These data indicate that in the EH rat baseline SNA is dependent on both the hypothalamus and AT1 receptors. Following chronic dehydration, the regulation of SNA transfers to the medulla oblongata, particularly the cNTS. This plasticity seems to be mediated via activation of the AP1 transcription factor; if AP1 transcription factor activity is blocked in NTS during dehydration, then control of sympathetic activity reverts back to forebrain regions.