1. 1. The time sequence of right atrial and left atrial depolarization as a guide to the origin of ectopic atrial beats was studied in 46 patients with heart disease: in 36 patients after electrical stimulation of known areas of the two atria and in 10 patients with ectopic atrial rhythms. A direct right atrial and a bipolar esophageal lead were recorded simultaneously with one or more limb and chest leads on a photographic recorder. A catheter-pacemaker assembly was used for electrical stimulation of the atria. 2. 2. Right atrial depolarization always preceded the left by an average of 0.04 sec. in all instances of normal sinus rhythm and after stimulation of the right atrium. Conversely, the left atrial component of the P wave occurred before the right by an average of 0.037 sec. when the wall of the left atrium was stimulated. 3. 3. In retrograde P waves ascending through the atrioventricular junction after stimulation of the ventricles, the right and left atria depolarized either simultaneously or in quick sequence, with an interatrial conduction time not exceeding 0.03 sec. 4. 4. Stimulation of the lower regions of the interatrial septum depolarized both atria very rapidly in a manner analogous to activation of the atria by retrograde P waves. 5. 5. It is concluded that the sequence of right and left atrial depolarization is a useful and possibly indispensable measure for determining the origin of the P waves. More refined studies may eliminate the overlapping encountered in the present investigation and render this technic an essential component in the study of atrial electrophysiology. © 1969.