Hypercholesterolaemia is an associated risk factor for cardiovascular disorders, including atherosclerosis and hypertension, and is thought to contribute to endothelial dysfunction. The role of cholesterol in the modulation of vascular endothelial and smooth muscle signalling has previously been demonstrated via its removal from membranes using agents such as methyl-β-cyclodextrin. However, to date there has been limited investigation into the effects of increased serum cholesterol concentration on vascular function. In this study, wire myography was employed on sections of thoracic aorta taken from the apolipoprotein-E knock-out mouse model (ApoE-/-) of hypercholesterolaemia. The tissue was equilibrated at 5mN of resting tension for 1hr before endothelial-dependent relaxation was assessed by exposure to 10µM acetylcholine, after pre-constriction with 10µM phenylephrine. Data are expressed as means ± S.E.M., compared by one-way ANOVA, unless stated. ApoE-/- mice fed a normal chow diet (8 weeks), showed no change in endothelial dysfunction when compared to age and strain matched controls (42.5±10.5 vs. 45.1±4.4% relaxation, respectively). However, feeding with a high fat diet (HFD; 8 weeks) caused a significant increase in thoracic aorta endothelial-dependent relaxation in both ApoE-/- (69.4±3.0 vs. 45.1±4.45% relaxation; p<0.05, n=7,8) and control mice (75.3±4.5 vs. 45.1±4.4% relaxation; p<0.01, n= 7,8) compared to the chow fed controls. This augmented response was not maintained after further high fat feeding to 16 weeks (51.6±12.1 vs. 45.1±4.4% relaxation; n=4,8). The transient increase in endothelial-dependent relaxation in high fat fed control mice was preserved after incubation with a nitric oxide synthase inhibitor (LNNA (N-nitro-L-arginine) 50µM; 24.7±6.0 vs. 50.4±8.0% relaxation; chow vs HFD, post-treatment; p<0.05, unpaired t-test, n = 4) but was reduced after incubation with a cyclooxygenase inhibitor (indomethacin 10µM; 60.2±4.3 vs. 71.8±3.8% relaxation; chow vs HFD, post-treatment; n=4). These data suggest that high fat feeding results in a transient increase in endothelial-dependent relaxation that is mediated by cyclooxygenase signalling. It is widely known that cyclooxygenase activity is modulated by inflammation; therefore, it is possible that the response observed may represent a compensatory vascular reaction to inflammation induced by high fat feeding.