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Diffuse impairment of ventricular function after cardiac surgery may be related to the generation during reperfusion of the myocardium of free radicals derived from oxygen. Fifteen patients undergoing elective coronary bypass surgery were studied by previously described assays for peroxidised lipids and for isomerised lipids which were used as indices of free radical activity. Serial blood samples were obtained from systemic arterial, mixed venous, and coronary sinus catheters before, during, and after the ischaemic period. The patients underwent coronary artery surgery on cardiopulmonary bypass with a membrane oxygenator, relative hypothermia 30-34 degrees C, and intermittent cross-clamping of the aorta. During the ischaemic periods there were no significant changes in the indices of free radical activity. During the reperfusion phase there was a significant increase in free radical indices in arterial and mixed venous blood. A small rise in free radical indices in coronary venous blood was not statistically significant. These data indicate that free radical activity is increased in patients shortly after the cessation of cardiopulmonary bypass. The pattern of distribution between the different sampling sites suggests that much of the observed increase in isomerised and peroxidised lipids originates from tissues other than the myocardium.