One hundred consecutive patients scheduled for coronary artery bypass grafting were randomized to receive either blood or crystalloid cardioplegia. Successful Holter monitoring for rhythm disturbances was done before and immediately after the operation in 83 cases. With both modes of cardioplegia there were increases in the occurrence of rhythm disturbances postoperatively. The increases were mostly statistically significant. There was no intergroup differences in the occurrence of arrhythmias. The association between these disturbances and cross-clamp times, myocardial temperatures during cross-clamping, myocardial fibrillation times during and after cross-clamping, CK-MB values and perioperative infarction all indicated ischaemia or incomplete myocardial protection as a major cause of the immediate postoperative rhythm disturbances.