AUTHORS' SYNOPSIS: An analysis was made of the electrocardiograms recorded daily from 20 dogs which received heterotopic cardiac allografts. The features associated with rejection were identified, the times of appearances of the features relative to the time of graft failure were noted, and the value of different lead and electrode systems evaluated. Recordings were taken using skin electrodes in all dogs and epicardial electrodes in two of the dogs. One other dog was used as a control with epicardial electrodes on its own heart. The relative merits of the recording systems are discussed and the use of epicardial leads is recommended. Emphasis is placed on the care required in applying the information obtained from these results and those in the literature to cardiac transplantation in man. The most significant features associated with acute, unmodified, rejection in this study were a decrease in the QRS voltage (20/20 cases), the appearance of bradycardia (18/20 cases), terminal arrhythmias (17/20 cases), significant S-T segment deviation (17/20 cases), and prolongation of the Q-T interval (15/20 cases). The clinical value of these features is limited by the fact that none of them alone is pathognomonic for rejection, that they occur late in the rejection process, and that the degree of QRS voltage depression needs to be large before it is significant.