Five patients with second degree intraatrial block are presented. The first three cases had a sick sinus syndrome with sinus bradycardia, broad P waves and episodes of atrial flutter or fibrillation. In these patients a Wenckebach phenomenon could bei elicited by atrial stimulation at a critical driving rate between the stimulated site and the recording electrode. In the first patient this conduction disturbance was obtained at several right atrial stimulation sites. The block could be elicited in the other two patients only in a limited area of the right atrium respectively only by left atrial pacing. In the remaining two patients an atrial tachycardia with block was observed. The intraatrial conduction disturbance was manifested as an exit block around the ectopic pacemaker. In one patient the tachycardia was induced by digitalis intoxication. In the other patient no etiologic factor of the tachycardia could be found. While the first three patients presented intraatrial conduction disturbances already in sinus rhythm, the last two cases showed after recovery from the atrial tachycardia P waves of normal duration and configuration.