Abstract—Prostaglandin (PG) E₂ has an established role in the regulation of vascular tone and reactivity. The present study examined the role and mechanism of microsomal PG synthase-1 (mPGES-1) in vascular response to angiotensin II (Ang II) infusion. A 7-day Ang II infusion at 0.35 mg/kg per day via osmotic minipump had no obvious effect on mean arterial blood pressure in mPGES-1^+/+ mice but induced a marked hypertensive response in mPGES-1^-/- mice, associated with a parallel increase in urinary 8-isoprostane excretion and aortic NADPH oxidase activity and mRNA expression of p47^phox, gp91^phox, and Nox1. The hypertension in mPGES-1^-/- mice was completely prevented by Tempol treatment and was fully restored on termination of the antioxidant. Apocynin induced a similar blood pressure–lowering effect as Tempol. The Ang II infusion induced mRNA expression of mPGES-1, as well as mPGES-2 and cytosolic PGE synthase in the aortas as assessed by real-time RT-PCR. Immunohistochemistry revealed remarkably enhanced immunoreactivity of mPGES-1 mostly in vascular smooth muscle cells. In cultured vascular smooth muscle cells, Ang II exerted a direct stimulatory effect on reactive oxygen species production, NADPH oxidase activity, and expression of p47^phox, gp91^phox, and Nox1 that were all inhibited by PGE₂. The -/- mice also exhibited enhanced renal hemodynamic response to acute Ang II infusion at 150 nmol/kg per minute via a jugular vein over a period of 40 minutes. These results suggest that mPGES-1–derived PGE₂ buffers Ang II–induced vasoconstriction via inhibition of NADPH oxidase–dependent reactive oxygen species production.