Insulin resistance is now well established as an independent risk factor for the development of type 2 diabetes and cardiovascular atherosclerosis. Consistent with this humans with insulin resistance such as obese, type 2 diabetic or subjects of South Asian origin are at substantially increased risk of cardiovascular atherosclerosis and its complications. Most studies have examined atherogenesis in models of severe insulin resistance or diabetes. However, by the time type 2 diabetes is diagnosed, these patients already demonstrate a significant atheroma burden. Furthermore, recent studies suggest that even in adolescence, relative insulin resistance may increase cardiovascular risk. Future therapies must therefore target progression of vascular dysfunction in mildly insulin resistant states prior to the development of diabetes. In the current work we studied the impact of very mild insulin resistance on endothelial function during ageing from adolescence through to adulthood. Endothelium dysfunction is a hallmark of insulin resistant conditions and is a key step in the initiation and progression of atherosclerosis. The term endothelium dysfunction embraces a number of abnormalities amongst which reduction in bioavailability of the antiatherosclerotic signalling molecule nitric oxide (NO)is of particular importance. The present series of studies used a murine model of insulin resistance at a global level to explore the temporal changes in NO bioavailability as mice moved from adolescence to adulthood. We used mice with haploinsufficiency for the insulin receptor (IRKO) a model of mild insulin resistance with preserved glycaemic control. We previously demonstrated that 2 month-old (Young) IRKO mice have preserved vasorelaxation responses to acetylcholine (Ach). We have now studied the effect of ageing on endothelium function in male IRKO mice compared to wildtype littermate controls. Despite no significant deterioration in glucose homeostasis, 6 month-old (Adult) IRKO mice had marked blunting of Ach-mediated vasorelaxation (IRKO Emax 66+/-5% versus wildtype 87+/-4%, p<0.01). This was restored by the superoxide dismutase mimetic MnTMPyP (IRKO Emax to Ach with MnTMPyP 85+/-5%). Dihydroethidium fluorescence of aortae and isolated coronary microvascular endothelial cells demonstrated a substantial increase in endothelium-derived reactive oxygen species in IRKO mice. These data demonstrate that mild insulin resistance is a potent substrate for accelerated endothelium dysfunction during ageing, and also suggests that reduction of endothelial superoxide production may preserve endothelium function over time.