Electrophysiologic studies with recordings of sinus node electrograms were performed in two patients with bradycardia-tachycardia syndrome. In both patients, the rest electrocardiogram showed apparent sinus bradycardia. Patient 1 had frequent paroxysms of atrial tachycardia with long pauses of up to 10 seconds; Patient 2 had paroxysmal atrial flutter and atrial pauses of up to 8 seconds. Multiple, repetitive, low frequency deflections, with a cycle length ranging from 730 to 960 ms in Case 1 and 570 to 750 ms in Case 2, suggestive of sinus node electrograms, were recorded at a critical area at the junction between the superior vena cava and the right atrium. These low frequency deflections had no relation to spontaneous junctional beats or the spontaneous atrial beats that showed high frequency deflections on the atrial electrogram. However, they could be suppressed by spontaneous or paced atrial beats. Pharmacologic interventions in Case 2 showed that the cycle length of the low frequency deflections shortened after administration of isoproterenol and did not change after propranolol or atropine. Thus, complete sinoatrial exit block with intact entrance conduction can occur in patients with bradycardia-tachycardia syndrome. Under such circumstances, the surface electrocardiographic manifestation of sinus bradycardia may not be of sinus origin.