Arteries from hypertensive animals and humans have increased spontaneous tone. Increased superoxide anion (superoxide) contributes to elevated blood pressure (BP) and spontaneous tone in hypertension. The association between the extracellular signaling-regulated kinase 1/2 (ERK1/2)-mitogen-activated protein kinase (MAPK) signaling pathway and generation of superoxide and spontaneous tone in isolated aorta was studied in angiotensin II (Ang II)-infused hypertensive rats (HT). Systolic BP, phosphorylation of ERK, aortic superoxide formation and aortic spontaneous tone were compared in sham normotensive and HT rats. Infusion of Ang II (0.5 mg/kg/day for 6 days) significantly elevated the systolic BP (p<0.01). The phosphorylation of ERK1/2 versus total ERK1/2 in thoracic aorta was enhanced and superoxide was increased in the HT versus the sham group (p<0.01). Spontaneous tone developed in HT group, but not in the normotensive group. MEK1/2-ERK1/2 signaling pathway inhibitors, PD98059 (10µmol/L) and U0126 (10µmol/L) significantly reduced the phosphorylation of ERK1/2, superoxide generation (p<0.01) and spontaneous tone (p<0.01) in HT. These findings suggest that Ang II infusion induces the production of superoxide and spontaneous tone and that both are dependent on ERK-MAPK activation. In endothelium-denuded aorta, however, MEK1/2 inhibitors did not inhibit the spontaneous tone, even though they significantly reduced superoxide generation similar to endothelium-intact aorta. These data suggest that inhibition of ERK 1/2 signaling pathway, via PD98059 or U0126, may regulate spontaneous tone in an endothelium-dependent manner. In conclusion, these findings support the importance of the ERK1/2 signaling pathway in modulating vascular oxidative stress and subsequently mediating spontaneous tone in HT.