The bolus intravenous injection of a novel medium-chain triglyceride:fish oil emulsion (MCT:FO) was recently proposed as a tool to provoke a rapid and sustained increase of cell phospholipid content in long-chain polyunsaturated omega3 fatty acids, for instance in selected subjects prior to anesthesia and surgery. In this study, therefore, the possible protective effect of MCT:FO upon aortic endothelial function was investigated in both normal and diabetic rats. The animals were injected intravenously 20 h before sacrifice with 1.0 ml of either saline, MCT:FO or a control medium-chain triglyceride:long-chain triglyceride emulsion. The vasomotor response of isolated aortic rings was then explored by assessing the relaxation provoked by increasing concentrations of acetylcholine in rings contracted with phenylephrine. Such measurements were performed before and after exposure of the aortic rings to suitable concentrations of oxidized LDL. In both normal and diabetic rats, the prior injection of the MCT:FO emulsion protected the aortic rings against the deleterious effect of oxidized LDL. In the diabetic rats, a beneficial effect of the MCT:FO emulsion was even observed prior to exposure of the aortic rings to oxidized LDL. These findings support the view that this novel procedure is indeed appropriate to protect endothelial function against oxidative stress.