An increase in plasma osmolality induced by either hypertonic saline infusion or dehydration causes an increase in sympathetic nerve activity (SNA). This is mediated, in part, by activation of sympathoexcitatory neurones in the paraventricular nucleus of the hypothalamus (PVN). It has been reported that in the anaesthetized rat the increase in sympathetic nerve activity (SNA) due to hypertonic saline infusion is dependent upon activation of AT₁ receptors in the PVN (Chen & Toney, 2001). However, the mechanisms that underlie the increase in SNA after dehydration remain to be fully established. In the present study using an in situ approach, we investigated the sequential effects of systemic administration of Losartan (20 μM, AT₁ receptor antagonist) and pre-collicular transection (to remove PVN) on the evoked thoracic SNA response to increased plasma osmolality caused by either acute infusions of hypertonic NaCl or after water deprivation for 3 days. Experiments were performed in the in situ working heart-brainstem rat preparation (Paton, 1996). Under deep halothane anaesthesia (assessed by an absence of a limb withdrawal reflex to noxious pinching), the rat was transected below the diaphragm and decorticated to make insentient. The rostral portion of the rat was 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 rats (290 mOsmol perfusate), systemic application of Losartan (20 μM) reduced baseline SNA by 20 ± 3% (p<0.05, t-test; n=5). Subsequent pre-collicular transection further reduced SNA (44 ± 2% from baseline). Transiently raising the perfusate osmolality to 380 mOsmol for 40 s in euhydrated rats increased SNA by 40 ± 5% (P<0.05, n=5). However, in the presence of Losartan this sympathoexcitatory response was attenuated greatly (i.e. a residual increase of 12 ± 5%, P<0.05), with no further attenuation observed after pre-collicular transection. In contrast, in the dehydrated rats perfused with hyperosmotic Ringer’s solution (340 mOsmol), Losartan and subsequent pre-collicular transection failed to reduce significantly SNA (-3 ± 7% and -12 ± 8% from baseline, respectively; n=5). These data indicate that in the euhydrated rat the sympathoexcitatory response induced by hypertonic stimulation is dependent on both the PVN and AT₁ receptors. In contrast, following chronic dehydration, regulation of the SNA does not appear to be dependent upon either supra-pre-collicular structures or AT₁ receptors.