Increased cardiovascular risk after mercury exposure has been described but the underlying mechanisms are not well explored. We analyzed the effects of chronic exposure to low mercury concentrations on endothelium-dependent responses in aorta and mesenteric resistance arteries (MRA). Wistar rats were treated with HgCl₂ (1^st dose 4.6 µg/kg, subsequent dose 0.07 µg/kg/day, i.m, 30 days) or vehicle. Blood levels at the end of treatment were 7.97±0.59 ng/ml. Mercury treatment: 1) did not affect systolic blood pressure; 2) increased phenylephrine-induced vasoconstriction; 3) reduced acetylcholine-induced vasodilatation and; 4) reduced in aorta and abolished in MRA the increased phenylephrine responses induced by either endothelium removal or the NO synthase (NOS) inhibitor L-NAME (100 µM). Superoxide dismutase (SOD, 150 U/ml) and the NADPH oxidase inhibitor, apocynin (0.3 mM), decreased the phenylephrine-induced contraction in aorta more in mercury-treated rats than controls. In MRA, SOD did not affect phenylephrine responses; however, when coincubated with L-NAME, the L-NAME effect on phenylephrine response was restored in mercury-treated rats. Both apocynin and SOD restored the impaired acetylcholine-induced vasodilatation in vessels from treated rats. eNOS expression did not change in aorta but was increased in MRA from mercury-treated rats. Vascular O₂^- production, plasmatic malondialdehyde levels and total antioxidant status increased with the mercury treatment. In conclusion, chronic exposure to low concentrations of mercury promotes endothelial dysfunction as a result of the decreased NO bioavailability induced by increases in oxidative stress. These findings offer further evidence that mercury, even at low concentrations, is an environmental risk factor for cardiovascular disease.