SUMMARY In the course of electrophysiological evaluation of six patients with sick sinus syndrome, two patients with chronic conduction system disease and four patients with paroxysmal supraventricular tachycardia or atrial flutter-fibrillation, the phenomenon of concealed conduction within the atrium (At) was repeatedly observed. One pair of intracardiac electrodes was used to deliver electrical stimulus (St) to the high right At and two additional pairs of electrodes were utilized to record high right and low septal right atrial electrograms. The interelectrode distances were 10 mm apart. In all 12 patients, high right atrial capture could be accomplished at a pacing rate of > 200 beats/min. Concealed intra-atrial conduction was evident when the intra-atrial conduction time of the propagated St during 2: 1 St-At block was more than 40 msec longer than that during 1: 1 St-At conduction at half the St frequency. This indicated that the nonpropagated St during 2: 1 St-At block partially penetrated the At and in turn, delayed conduction of the subsequently propagated St. Further observations revealed that a gradual increase in the St frequency resulted in a progressive prolongation of the intra-atrial conduction time and a shifting of 2:1 to 3:1 St-At block (alternating Wenckebach periodicity in the At) in five patients. These findings clearly demonstrate the occurrence of concealed conduction in the human At.