['The present study investigated whether or not calcitriol, an active form of vitamin D, protects against renovascular dysfunction in hypertension and, if so, whether or not such protection alters the expression of key proteins involved in that dysfunction.', 'Changes in isometric tension showed that the impaired endothelium-dependent relaxations in renal arteries of hypertensive patients were enhanced by 12 h in vitro treatment with calcitriol. Dihydroethidium fluorescence revealed an elevated level of reactive oxygen species (ROS) in these arteries which was reduced by calcitriol. Immunofluorescence showed that calcitriol treatment reduced the expression of AT(1)R, NOX-2, NOX-4, and p67(phox) and increased that of superoxide dismutase (SOD)-1. Twelve-hour exposure to calcitriol prevented angiotensin (Ang) II-induced increases in ROS and the over-expression of NOX-2, NOX-4, and p67(phox) in renal arteries from normotensive patients. A specific antagonist of the human vitamin D receptor (VDR), TEI-9647, abolished these effects of calcitriol. Both in vitro exposure to and chronic in vivo administration of calcitriol enhanced relaxations to acetylcholine and abolished exaggerated endothelium-dependent contractions in renal arteries of normotensive rats pre-exposed to Ang II or harvested from spontaneously hypertensive rats (SHR). Reactive oxygen species levels and expressions of AT(1)R, NAD(P)H oxidase subunits, SOD-1, and SOD-2 in SHR arteries were normalized by the chronic treatment with calcitriol.'] Does calcitriol protect renovascular function in hypertension by down-regulating angiotensin II type 1 receptors and reducing oxidative stress?