The number of granulocytes, their ability to generate superoxide anion (O2-) and the activities of Cu, Zn--superoxide dismutase (SOD-1), glutathione peroxidase (GSH-Px), catalase (CAT) as well as malonyldialdehyde (MDA) concentrations in erythrocytes in the blood extracted from the venous sinus and aorta under coronary artery bypass were examined with the use of St. Thomas Hospital cardioplegic solution. Specimens at the peak of ischemia of the right atrium for ultrastructural examination of the endothelial cells of capillary vessels and sarcomers were taken. The blood was obtained during cardiopulmonary bypass (CPB) before the aorta clamping and immediately after aorta declamping (peak of ischaemia) between 1-3 minute and 10-13 minute of reperfusion. Increase of the number of granulocytes both in the coronary sinus and aortal blood at all examined intervals as well as decrease in the number of ones in sinus compared with aortal blood was noted. The ability to produce superoxide anion radical decreased at the peak of ischemia and during reperfusion. The activity of SOD-1 was lower both after the period of ischemia and reperfusion. The increase in aortal blood activity during reperfusion was characteristic of GSH-Px; the activity was higher in the blood sample from the coronary sinus taken during ischemia and initial reperfusion. With the exception of the initial reperfusion the activity of CAT diminished in all observed cases. MDA concentration did not demonstrate any significant changes with the exception of the initial reperfusion in the aortal blood and later towards the end of reperfusion in the blood from the coronary sinus. Ultrastructural studies indicated overhydration of the cells both in the endothelium and the intercellular space. The obtained data demonstrate that the applied cardioplegic solution protects the myocardium from harmful effects of reactive oxygen species produced as a result of ischemia and reperfusion.