Abstract: Aim\ Nociceptin/Orphanin FQ is a newly discovered endogenous ligand for the opioidlike Gprotein coupled receptor（ORL 1）.\ This study is to investigate the changes of nociceptin in patients with essential hypertension（EH） and its hypotensive mechanism.\ Methods\ The level of nociceptin in plasma from EH is studied by radioimmunoassay with healthy population as control.\ In additionally,the effect of nociceptin on the isolated aortic ring tissues are assessed by EDRF/NO（endothelium derived relaxing factor/nitric oxide） production and NO synthase（NOS） activity.\ Results\ The level of plasma nociceptin in EH is much higher than that in normal control（1547±214 ng/L vs 883±568 ng/L,P＜005）.\ Nociceptin can stimulate the aortic ring tissues′ NO production and NOS activity in a dosedependent manner.\ Conclusion\ The above results suggest that endogenous nociceptin possesses biologic activity outside CNS and may be a new vasoactive substance that may contribute to the regulation of blood pressure.