The increasing number of heart transplant patients requires that physicians be able to recognize the electrocardiographic (ECG) and electrophysiologic properties of cardiac allografts. Cardiac allografts are characterized by modifications of resting ECGs and frequent arrhythmias in the postoperative period, and the loss of autonomic nervous control illustrated by permanent tachycardia and loss of heart rate variability during 24-hour ambulatory ECG recording. Some clinical and experimental observations suggest a mid-term reinnervation of the cardiac allograft, but this requires histologic confirmation. The electrophysiologic characteristics of the denervated myocardium are similar to those of the innervated myocardium at rest. However, supersensitivity to circulating catecholamines has been observed in cardiac allografts as in experimentally denervated hearts, which is responsible for a progressive increase in heart rate during exercise and a slow decrease during recovery. Supersensitivity of the denervated heart to acetylcholine may explain the high prevalence of donor sinus dysfunction due to impairment of its automaticity. More often, the sinus node dysfunction is transient and can be treated with an adenosine antagonist, such as theophylline, before permanent implantation of a pacemaker. In the case of pacemaker implantation, synchronization of the donor atria with the recipient atria is desirable, and an endocardial lead implantation is preferred. Several electrophysiologic changes have been observed during acute cardiac allograft rejection. From experimental studies, the most important of these are the disturbance of conduction in the atria and the atrioventricular node and a decrease in the amplitude of the ventricular potential. Initial studies on isolated myocytes show profound changes in membrane conductance during experimental cardiac rejection. The development of new noninvasive detection methods of cardiac allograft rejection, such as intramyocardial voltage electrogram monitoring and high-resolution ECG, could help early diagnosis.