BACKGROUND: We have previously demonstrated that chronic treatment with the hexapeptide angiotensin (Ang) IV significantly improved endothelium-dependent vasorelaxation in isolated aorta obtained from young adult apolipoprotein E-deficient (ApoE(-/-)) mice. The current study aimed to investigate whether this effect is evident using ApoE(-/-) mice with established atheroma. MATERIALS AND RESULTS: ApoE(-/-) mice fed a high-fat diet for a period of 26 weeks displayed significantly impaired endothelial function compared to wild-type mice. Importantly, 2-week treatment with Ang IV, significantly improved endothelial function despite the abundance of atherosclerotic lesions. Endothelial nitric oxide synthase immunoreactivity was significantly increased, whereas superoxide levels assessed using dihydroethidium staining were concomitantly decreased, in aortic cross-sections taken from Ang IV treated mice compared with vehicle treated ApoE(-/-) mice. [(125)I] Ang IV autoradiographic analysis revealed an upregulation of AT(4)Rs in ApoE(-/-) mice fed a high-fat diet for 26 weeks compared to 8 weeks. Co-treatment with an AT(4)R antagonist, divalinal-Ang IV or an AT(2)R antagonist, PD123319 significantly abrogated these Ang IV-induced vasoprotective effects. CONCLUSIONS: We have demonstrated that the improvement in endothelial function following chronic Ang IV treatment is conserved at an advanced stage of atherosclerosis. Consistent with previous findings using an early-stage model, this vasoprotective effect of Ang IV was AT(4)R- and/or AT(2)R-mediated, involving increased nitric oxide bioavailability.