Mercury chloride exposure for 30 days decreases NO bioavailability and increases oxidative stress. However, the mechanisms underlying the effects of mercury on the cardiovascular system are not completely understood, and it is not known if they are dose-dependent or if some concentrations have no harmful effects. Thus, we investigated the effects of chronic exposure to doses low (half) and high (2.5-fold higher) than that needed to obtain 29 nmol/L of HgCl2 on the vascular function. Three-month-old male Wistar rats received intramuscular (i.m.) HgCl2 for 30 days and were divided in three groups: lower (Low Hg); higher (High Hg); and saline was used as the control. High Hg exposure increased the contractile response to phenylephrine (PHE) in aortic rings, but Low Hg reduced it. The hyporesponsiveness in the Low Hg rats was blunted by endothelial denudation and NOS inhibition with l-NAME (100μmol/L). The phosphorylated-eNOS/eNOS protein ratio increased in the aortas of Low Hg rats. In the High Hg group, endothelial denudation increased the PHE-induced contractions, while l-NAME had no effects and indomethacin (10μmol/L), losartan (10μmol/L) and apocynin (30μmol/L) reduced this response. In the High Hg group, protein levels of the NADPH oxidase subunit gp91phox and cyclooxygenase-2 increased. Our results support previous suggestions that High Hg increases oxidative stress that might activate an inflammatory cascade and the renin-angiotensin system. However, very low Hg concentrations below the level considered safe still reduced vascular reactivity, suggesting the need for special attention to continuous exposure as a putative cause of increased cardiovascular risk.