Abstract: To observe the changes of surgical reperfusion injury, plasma malonyl dialdehyde （MDA）, diened with conjugated double bond （DCDB）, whole blood superoxide dismutase （SOD） activity and plasma manganese-containing SOD （Mn-SOD） activity were determined during cardiopulmonary bypass （CPB） in 5 cases undergoing cardiosurgery. The results showed that reperfusion period on CPB was associated with a significant increase in the products of lipid peroxidation, MDA and DCDB （P＜0.05 for each）. However, it indicated that SOD of suppress superoxide anion （O₂^-） fell signiticantly （P＜0.05）. Of interest was the finding that plasma Mn-SOD activity rose signiticantly （P＜0.05）during reperfusion period.