The endothelial dysfunction seem to be responsible for occurrence of many diseases of the circulatory system. In the hereby elaboration an effort was undertaken to work out a evaluation method of a degree of the endothelial dysfunction in conditions of the oxidative stress caused by the activation of oxidase NAD(P)H. The first part of research was carried out on isolated aortas of control rats (WKY) and rats with idiopathic hypertension (SHR). The second part was carried out on arteries and veins obtained from 160 patients after the surgical procedure of coronary arteries bridging. The dysfunction degree of the endothelium was evaluated by comparison of vasomotor answers of vessels to NADPH and acetylocholine by a technique of the organ bath. There was performed for the same vessels measurement of O2- production by the chemiluminescence. It was proved that at SHR production of O2- is higher than during control, and the main source of O2- is oxidase NAD(P)H. There were observed contrary answers of vessels after NADPH: contractions at SHR (depended on O2- production) and relaxations at WKY (depended on NO and). Similar observations were proved in human vessels - aortas of SHR behaved as human vessels with endothelial dysfunction. In the human vessels the endothelial dysfunction degree correlates with production of O2- and a number of atheromatosis risk factors.