In order to evaluate perioperative electrical cardiac disturbances and ST segment changes, 42 patients (38 M, 4 F, aged 57 +/- 6 ys) were studied using 24-hour Holter monitoring before, during and after coronary bypass surgery. In the 4-6 hours before cardioplegic arrest, 38% of patients had ST segment changes. No patient and malignant arrhythmias. The injection of cold cardioplegic solution was followed by bradycardia, ventricular tachycardia, ventricular fibrillation and isoelectric line within 2-4 minutes. After aortic declamping, 30 patients were defibrillated. Impulse formation and conduction disturbances, found in 55% of patients, solved themselves in 1 to 60 minutes. Bundle branch block continued in just 4 cases. A total of 59% of patients had ST segment elevation for 14 +/- 14 minutes and 19% had ST segment depression for 19 +/- 20 minutes. Successive transient ST segment changes were detected in 38% of patients. Sustained ventricular tachycardia occurred during 2 ischemic episodes. Impulse formation and conduction disturbances were not related to the duration of cardiac arrest or ventricular fibrillation, but were more frequent and lasted longer in patients with incomplete revascularization. Transient ST segment depression far from aortic declamping correlated with preclamping ischemia. Transient ST segment elevation correlated with incomplete revascularization. We concluded that ECG signs of intraoperative damage were reversible. Moreover, perioperative transitory ischemia was frequent but could be prevented by coronary active drug administration. On the other hand incomplete revascularization was associated with electrical disturbances and ischemia.