Fourteen long term survivors with orthotopic (recipient heart replaced by donor heart) and nine with heterotopic cardiac transplants (recipient heart retained) had 24 hour ambulatory electrocardiographic monitoring to detect ventricular arrhythmias. Arrhythmia was uncommon in the patients with orthotopic transplants; none of them had more than one extrasystole per hour. In the patients with heterotopic cardiac transplants the recipient's own heart showed significantly more frequent ventricular arrhythmias than the corresponding donor heart: abnormal complexes (mean/24 h) 4583 vs 42.7; extrasystoles 1772 vs 17.8; pairs 121 vs 0.8. There was no relation between the abnormal ventricular activity of the two hearts in the patients with heterotopic transplants on a beat by beat, hourly, or 24 hour basis. There was no consistent diurnal variation in the frequency of the abnormal ventricular beats after cardiac transplantation. The occurrence of ventricular arrhythmia was unrelated to the interval from operation to the study. In long term survivors of cardiac transplantation the denervated heart shows a little ventricular ectopic activity even when compared with normal hearts. In patients with heterotopic transplants ventricular arrhythmias commonly occur in the recipient's own heart; these are probably related to the underlying severity of the original disease.