Background: Angiotensin II (Ang II) stimulates the production of reactive oxygen species and activation of pro-inflammatory cytokines leading to endothelial dysfunction. We hypothesize that the anti-inflammatory cytokine interleukin-10 (IL-10) counteracts the impairment in endothelium-dependent ACh relaxation caused by Ang II. Methods and Results: Aortic rings of C57BL/6 mice were incubated in Dulbecco modified Eagle's medium (DMEM) in the presence of either vehicle (dH₂O), Ang II (100 nmol/L), recombinant mouse IL-10 (300 ng/mL) or in the presence of both Ang II and IL-10 for 22 hrs at 37° Celsius. Following incubation, rings were mounted in a wire myograph to assess endothelium-dependent vasorelaxation to cumulative concentrations of acetylcholine (ACh). Overnight exposure of aortic rings with Ang II resulted in blunted ACh-induced vasorelaxation compared to untreated rings (E_max = 44±3% versus 64±3% respectively; P<0.05). IL-10 treatment significantly restored this impairment in relaxation (63±2%). Additionally, the NADPH oxidase inhibitor apocynin, restored the impairment in relaxation (E_max = 76±3%). Western blotting showed increased gp91phox expression (a subunit of NADPH oxidase) in response to Ang II. Vessels treated with a combination of Ang II and IL-10 showed decreased expression of gp91phox. Immuno-histochemical analysis showed increased gp91phox expression in Ang II treated vessels as compared to those treated combined with Ang II and IL-10. Conclusion: The anti-inflammatory cytokine IL-10 prevents impairment in endothelium- dependent vasorelaxation in response to long term incubation with Ang II via decreasing NADPH oxidase expression.