In this thesis I have examined the vascular reactivity of human peripheral small arteries in vitro in hypercholesterolaemia. I have also examined the effects of cholesterol lowering therapy and the addition of marine fish oils to the diet. Subcutaneous gluteal fat biopsies were taken from 18 hypercholesterolaemic patients and 16 age and sex matched controls. Subcutaneous small arteries were dissected and mounted on a wire myograph for isometric tension experiments. The hypercholesterolaemic patients showed impaired relaxation to endothelium dependent agents and a smaller impairment to an independent agent as compared to control. After successful cholesterol lowering therapy the vascular function had returned to normal on rebiopsy. A further 16 hypercholesterolaemic subjects and 12 controls underwent fat biopsy and were then randomised to receive either marine fish oil supplements or placebo for 3 months and then were restudied. Although serum lipids were unchanged endothelial function was improved in the hypercholesterolaemic patients given fish oils. My studies have demonstrated that vascular function is impaired in hypercholesterolaemia. I have demonstrated that the impairment is improved in vitro by the nitric oxide substrate L-arginine and dietary supplementation with fish oil. Restoring cholesterol to the normal range results in a restoration of function to control values. The studies further demonstrate the value of cholesterol reduction and marine fish oils in the treatment of vascular disease. They also raise important questions over the role of increased lipids on blood pressure homeostasis.