The renin-angiotensin system potently regulates blood pressure and fluid homeostasis. Whilst originally described as a circulating, humoral system, several tissue RAS exist, including those of the heart and brain. Physiological studies have shown that the predominant actions of angiotensin II (Ang II) in the brain involve cardiovascular regulation, consistent with the circulating RAS. Some studies suggest that the brain RAS functions and is regulated independently of the circulating system. The only known precursor of angiotensin peptides is angiotensinogen (AGT) and, in the brain, it is produced predominantly by astrocytes. Local upregulation of AGT in astrocytes of the brain has been observed in response to steroid hormones and osmotic stimulation. New evidence in cultured astrocytes suggests that activation of angiotensin type 1A receptors (AT1ARs) in astrocytes does not increase AGT gene expression. In this study, the hypothesis that AT1ARs activation negatively regulates AGT was tested in vivo. Recombinant adenoviral vectors were used to express the wild type or a constitutively active mutant (N111G) version of the AT1ARs in rat astrocytes. Primary astrocyte cultures from neonatal C57Bl6 mice were used to first validate the use of this virus in vitro. When cultured astrocytes were transduced with [N111G]AT1ARs, they exhibited a decrease in AGT gene expression after 72 hours (44.9% ± 13.4%, P=0.02, n=6). AGT gene expression was further reduced (24.5% ± 8.7%, P<0.01, n=6) 24 hours after administration of AngII (100nM). To determine whether AGT was similarly regulated by AT1ARs in vivo, male rats (250-350g, n=3) were anaesthetised with ketamine (60mg kg-1) and medetomidine (250μg kg-1, both i.m) and 1.3 × 107 viral particles of either the recombinant adenovirus encoding [N111G]AT1ARs or the wild type receptor control, were microinjected into the nucleus of the solitary tract. The rats were then allowed to recover for 5-6 days before histological analysis. A marked reduction in AGT immunoreactivity and gene expression was observed in astrocytes that expressed the constitutively active AT1ARs, but not in astrocytes expressing the wild type receptor. This demonstrated for the first time in vivo that AT1Rs signalling pathways can negatively regulate AGT production in astrocytes. An important consideration of these data is that by over-expressing [N111G]AT1ARs one has, in effect, transduced astrocytes with a G-protein coupled receptor that couples to the InsP3/Ca2+ signalling pathway. These results could, therefore, be extrapolated to any receptor present on astrocytes that couples to the same intracellular signalling pathways. Thus, whilst regulation of AGT may not occur via Ang II in astrocytes, a pathway that negatively regulates AGT expression both in vitro and in vivo has been identified.