To investigate the mutual relationship between angiotensin II (Ang II) and nitric oxide (NO) in paraventricular nucleus (PVN), Ang II receptor type Ia (AT1A), type Ib (AT1B), endothelial constitutive nitric oxide synthase (ecNOS), and neuronal constitutive nitric oxide synthase (ncNOS) mRNA levels of rat PVN were measured after unilateral carotid artery ligation. AT1A and AT1B mRNA levels were markedly elevated 6 hrs after unilateral carotid artery ligation. Losartan injection (10μg/0.3μl) into the PVN augmented of the increment of AT1A and AT1B mRNAs It also increased ecNOS gene expression. In addition, AT1B mRNA levels increased after N-nitro-L-arginine methyl ester (L-NAME) injection (50μg/0.3μl) into the PVN. These results suggest that Ang II and NO in the rat PVN may interplay, at least in part, through regulation of gene expression of ecNOS and AT1B, respectively.