Electrophysiologic studies, including intra-atrial recordings and atrial stimulation, were performed in two patients with suspected sick sinus syndrome. Premature atrial stimuli induced atrial flutter in both patients. The arrhythmia was concealed, i.e., it was recordable only by intracavitary electrogram and invisible on surface electrocardiogram. In one case, simultaneous atrial fibrillation could be recorded in a segment of the right atrium. In this patient, the rhythm on the surface electrocardiogram changed during the study to "upper nodal" rhythm, though the atrial electrogram showed continuation of A waves at the same rate as before during sinus rhythm. It seems that atrial changes, which are frequently encountered in sick sinus syndrome, are a predisposing factor for spontaneous or inducible concealed atrial arrhythmias.