The Mediterranean diet, which is abundant in antioxidants, is associated with a relatively low incidence of coronary heart disease (CHD). Olive oil and olives, containing the antioxidants hydroxytyrosol, oleuropein and tyrosol, are important components of this diet. In this study the effects of oxidative stress on the NO^· mediated relaxation of rat aorta and the protection by these antioxidants were determined. Cumene hydroperoxide (CHP) was used to mimic oxidative stress induced by lipid hydroperoxides, which is mediated by the formation of hydroxyl radicals (OH^·). CHP (300 µM) impaired the NO^· mediated relaxation of rat aorta by the acetylcholine receptor agonist carbachol (P<0.05). This was due to a reduction in NO^· production. A diminished NO^· mediated relaxation disturbs the vascular tone and leads to a rise in blood pressure, which is a well established risk factor for CHD. Hydroxytyrosol (10 µM) efficiently protected the aorta against the CHP induced impairment of the NO^· mediated relaxation (P<0.05). Oleuropein, tyrosol and homovanillic alcohol, a major metabolite of hydroxytyrosol, did not show any protection. Moreover, hydroxytyrosol was found to be a potent OH^· scavenger, which can be explained by its catechol moiety. Because of its amphiphilic characteristics (P_ow = 1.1), hydroxytyrosol will readily cross membranes and provide protection in both the cytosol and in membranes, including the water-lipid interface. Amphiphilic compounds, such as hydroxytyrosol, are generally well absorbed. The present study provides a molecular basis for the contribution of hydroxytyrosol to the benefits of the Mediterranean diet.