The paraventricular nucleus of the hypothalamus (PVN) plays a crucial integrative role in the regulation of cardiovascular and neuroendocrine systems, and body fluid homeostasis. Nitric oxide (NO) is known to stimulate GABAergic transmission in the PVN, and through these actions may tonically suppress sympathetic outflow1. However, the chronic role of NO in the PVN for regulating autonomic function remains to be established. To address this, we used a lentiviral vector system in order to knock down nitric oxide synthase (nNOS) levels in the PVN of conscious rats. Blood pressure (BP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were monitored continuously in Wistar rats (n=4) using telemetry devices implanted under isoflurane anaesthesia (1-2%, inhalation). RSNA was normalised to the maximal response elicited by activation of the nasopharyngeal reflex with cigarette smoke (100 normalised units, nu), and background noise level (0 nu)3. Following recovery and baseline recording of BP, HR and RSNA, rats were re-anesthetised with ketamine and dormitor (60mg.kg-1/250ug.kg-1 intraperitoneal injection) and the PVN microinjectioned bilaterally with a neurone-specific Tetracycline-controllable lentiviral vector3 expressing a short hairpin miRNA-30 (shRNA) interference system targeting endogenous nNOS. Recordings continued for a further 30 days. The construct was GFP-tagged, and successful placement and expression in the PVN was confirmed post-hoc. Knock-down of the nNOS gene resulted in an increase in BP beginning seven days after microinjection (95±5 to 105±5 mmHg,) and maintained for the full 30 day period of recording. HR was elevated from 311±20 to 327±13 bpm. Preliminary results indicate concurrent activation of the sympathetic nervous system, with SNA increasing to more than double baseline levels (from 7.3 to 16.7 nu by Day 30). These results suggest that NO activity in the PVN is a critical determinant of the chronic level of sympathetic drive in the rat.