It is known that in the presence of oxygen radicals, anti-atherogenic nitric oxide is converted into pro-atherogenic products, which increase lipid peroxidation. In this study, plaque-free atherosclerotic tissues (n=26), atherosclerotic plaques (n=26) and fetal tissues (n=2; as control) were evaluated. High nitrite, but low malondialdehyde, levels in non-atherosclerotic tissues may show the protective role of nitric oxide from atherosclerosis. In plaque-developed tissues nitrite levels were three times, and lipid peroxidation levels were 10 times, higher than non-plaque developed tissues. In the atherosclerotic plaque forming process, the role of nitric oxide can be discovered according to the lipid peroxidation of tissues. In conclusion, the results of this study show an inversely proportional relation between pro-and anti-atherogenic effects of nitric oxide in the pathogenesis of atherosclerotic vascular diseases. (C) 2003 Elsevier Ireland Ltd. All rights reserved.