Cardiac arrhythmias were recorded during 24 hours in 40 consecutive patients requiring open-heart surgery, using continuous Holter monitoring. Most patients developed both supraventricular and ventricular arrhythmias. Complex ventricular arrhythmias were detected in 31 patients (78%) and 2 patients suffered an immediate postoperative myocardial infarction. The following risk factors were considered: age, sex, type of heart disease, preoperative left ventricular ejection fraction, cardiopulmonary bypass and aortic clamping duration, length of anesthesia, dopamine administration, and maximal level of CK and CK-MB. Only dopamine administration, even in low renal dose, was associated with the number and severity of ventricular arrhythmias.