During cardiac transplantation the posterior and lateral portions of the recipient's right and left atria and a posterior rim of interatrial septum are left intact. Persistent electrophysiologic activity of these residual recipient atrial remnants has been manifest as P waves dissociated from the donor heart rhythm and recorded in both standard and intracardiac electrocardiograms. Physiologically appropriate changes in recipient atrial rate have occurred in response to the respiratory cycle, donor heart systole, atropine, tyramine, amyl nitrite, carotid sinus pressure, the Valsalva maneuver and exercise. In several patients synchronization of the recipient atrial rate with the donor heart rate has been observed during exercise when these rates approached a similar level. Fibrillation and flutter of the recipient atria, independent of the donor heart rhythm, have been observed in 3 recipients. In 4 patients studied by cardiac catheterization 1 year postoperatively mechanical effects of recipient atrial contractions were detected in both right- and left-sided pressure measurements. Recipient left atrial contraction occurring simultaneously with donor atrial systole increased left ventricular end-diastolic pressure by 2 to 4 mm Hg and produced significantly greater peak left ventricular and arterial systolic pressures as well as augmented left ventricular ejection times. In several patients, especially during episodes of threatened graft rejection, diastolic heart sounds related to recipient atrial contraction were noted clinically and recorded on phonocardiograms. Such recipient atrial gallop sounds simulated both atrial and ventricular gallop sounds of donor heart origin, and occurred in addition to both to produce hybrid diastolic gallop rhythms. The physiologic mechanisms and implications of recipient atrial behavior are discussed.