Abstract—Mas codes for a G protein–coupled receptor that is implicated in angiotensin-(1-7) signaling. We studied the cardiovascular phenotype of Mas-deficient mice backcrossed onto the FVB/N genetic background using telemetry and found that they exhibit higher blood pressures compared with controls. These Mas^-/- mice also had impaired endothelial function, decreased NO production, and lower endothelial NO synthase expression. Reduced nicotinamide-adenine dinucleotide phosphate oxidase catalytic subunit gp91^phox protein content determined by Western blotting was higher in Mas^-/- mice than in controls, whereas superoxide dismutase and catalase activities were reduced. The superoxide dismutase mimetic, Tempol, decreased blood pressure in Mas^-/- mice but had a minimal effect in control mice. Our results show a major cardiovascular phenotype in Mas^-/- mice. Mas-deletion results in increased blood pressure, endothelial dysfunction, and an imbalance between NO and reactive oxygen species. Our animals represent a promising model to study angiotensin-(1-7)–mediated cardiovascular effects and to evaluate Mas agonistic compounds as novel cardioprotective and antihypertensive agents based on their beneficial effects on endothelial function.