Mice heterozygous for a knockout of the insulin receptor gene (IRKO) represent a non-obese model of mild insulin resistance and demonstrate a 30% reduction in stimulation of the PI3kinase pathway in response to insulin(1). We recently demonstrated loss of the vascular response to insulin in young 8-week IRKO mice but preservation of endothelial function assessed by relaxation responses to acetylcholine(2). In this study, we sought to examine the effect of a mild defect in insulin signalling on endothelial function with ageing. We studied metabolic function and blood pressure in vivo, and vascular function in aortic rings ex vivo in young (2 months) and middle aged (6 months) IRKO mice and their wild type (WT) littermates as previously described (2). All animals were humanely killed. n=6-8 per group, data presented as mean±SEM, p<0.05 taken as significant, Student's t test and ANOVA used where appropriate. Systolic blood pressure (measured by tail cuff plethysmography) was significantly greater in IRKO mice than WT at 8 weeks (124±4 and 110±3 n=8, p=0.01). There was no significant change in blood pressure with ageing in either IRKO (124±4 and 119±6) or WT mice (110±3 and 106±6). At 6 months IRKO mice showed significantly augmented contraction to phenylepherine (Emax(g) 2 months 0.6±0.06; 6 months 0.96±0.02; p=0.04) and impaired acetylcholine mediated relaxation (91.9%±8 and 66.3%±7; p=0.01). IRKO acetylcholine responses were partially normalised by the SOD mimetic MnTMPyP (10μM; Emax pre 77.6±6 post 91±2; p=0.03). WT vascular responses were unchanged between 2 and 6 months. Despite the evolution of significant endothelial dysfunction, the metabolic phenotype of IRKO mice remained mild. Fasting blood glucose was similar in IRKO (8.4±0.4 and 7.8±0.4) & WT (9.1±0.6 and 8.2±0.5) of both ages, as were blood glucose responses during insulin tolerance testing. However, old IRKO mice demonstrate significantly greater blood glucose and increment in blood glucose 30 minutes after an intraperitoneal glucose challenge (1mg/g) than WT mice (peak blood glucose: 16.9 and 13.4; p=0.001; increment from baseline: 123%±16 and 68%±15; p=0.01 respectively). They also exhibit greater visceral adiposity (mesenteric fat pad mass: IRKO 1.18mg±0.21; WT 0.74mg±0.05; p=0.04) despite similar total body weights (28.2g±0.9 and 29.2g±1.2). We conclude that a mild defect in insulin signalling promotes significant premature endothelial dysfunction with ageing, despite minimal deterioration in metabolic phenotype. Our data supports a potential role for reactive oxygen species in this process.