The direct vasoactive effects of native and oxidatively modified low density lipoproteins as well as their effects on endothelium-dependent relaxations to 5-hydroxytryptamine were studied in isolated rings of pig right coronary artery. Slowly developing contractions were caused by native low density lipoproteins (100 micrograms protein/ml). The contractions were more pronounced in the absence than in the presence of the trace metal chelator, EDTA, and coincided with the formation of lipid peroxides during the response. The lipophilic antioxidant, butylated hydroxytoluene, prevented the oxidation of, and contraction to, native low density lipoproteins. Low density lipoproteins oxidized by exposure to copper contracted coronary arteries more rapidly with a threshold of only 1 micrograms protein/ml, but with a similar maximal contraction at 100 micrograms protein/ml. Superoxide dismutase inhibited the contraction to native low density lipoproteins, but not to oxidized low density lipoproteins. Catalase blocked contractions to both native and oxidized low density lipoproteins. Contractions to oxidized low density lipoproteins were unaffected by indomethacin, but were abolished by removal of the endothelium or by inhibitors of endothelium-derived relaxing factor. Oxidized low density lipoproteins but not native low density lipoproteins inhibited endothelium-dependent relaxations to 5-hydroxytryptamine. Thus, oxidized low density lipoproteins caused endothelium-dependent coronary artery contractions which are mediated by a hydroperoxide. Contractions to native low density lipoproteins are due to their oxidation in the organ chamber by the superoxide anion radical. Oxidized, but not native, low density lipoproteins impair normal endothelial cell vasodilator function in vitro. Oxidized low density lipoproteins, important in the pathogenesis of atherosclerosis, may directly contribute to the increased risk of vasospasm seen in hypercholesterolemia and atherosclerosis.