Ultrastructure of air-hematic barrier and efficiency of alpha-tocopherolum use for prophylaxis of toxic effect of oxygen used for artificial pulmonary ventilation (APV) and oxygenation of blood outside lungs were studied in 27 patients with mitral and aortal valve diseases and ischemic disease before the operation and on its stages. Macroscopically unchanged lung regions were studied before the heart-lung apparatus switching, 40-45 minutes after clamping the artery in conditions of disturbed blood circulation and 10 minutes after hemostation in pulmonary artery. 6 patients were treated with 100% oxygen for 30 minutes before APV finishing while the other 6 previously received alpha-tocopherolum. Disturbance of the structure of the outermost cell membrane in type I alveolocytes and endotheliocytes, widening of basal layers and their fibers and disorders in the osmiophilic lamellar bodies were established to determine the severity of condition of patients with mitral valve diseases.