Exogenous insulin therapy improves endothelial function in insulin resistant patients, indirectly indicating that nitric oxide synthase activity and NO production may be impaired. Insulin stimulates production of NO by activating a signaling pathway including insulin receptor substrate-1, phosphatidylinositol-3-kinase and protein kinase B (PKB/Akt). Angiotensin II type I (AT1) receptor-evoked oxidative stress is implicated in the inactivation of NO, impairing endothelium-dependent vasodilatation. Blocking the actions of Angiotensin II with an AT1 receptor antagonist (Losartan), has beneficial effects in patients with insulin resistance or type 2 diabetes mellitus. This study investigated whether elevated Angiotensin II influences myocardial insulin resistance, insulin signaling and NO production in a rat model of diet-induced obesity (DIO) by antagonizing the actions of the AT1 receptor with Losartan. Isolated, perfused hearts, Western blotting and flow-cytometric methods were utilized to determine myocardial function, expression and phosphorylation of key proteins and NO production, respectively. Results showed that hearts from DIO rats are insulin resistant (higher serine phosphorylation of IRS-1, lower insulin-stimulated phosphorylation of PKB/Akt and eNOS, lower NO production) and had poorer functional recovery and larger infarct development after ischaemia/reperfusion. Losartan improved the impaired functional recovery, and NO production and enhanced eNOS expression and phosphorylation and reduced infarct size in hearts from the DIO animals. Data obtained from Losartan treatment also revealed that Angiotensin II signaling modulates myocardial PKB/Akt expression. We conclude that Angiotensin II signaling exacerbates inhibition of NO production in insulin resistance and that this can be improved by AT1 antagonism.