Invasive electrophysiological studies were performed in 2 symptomatic patients with recurrent arrhythmias in which impulse formation presumably occured within atrioventricular nodal bypass tracts. Case 1 had ectopic beats arising within, or close to, the upper end of a left-sided atrioventricular nodal bypass tract of the type described by Brechenmacher. In addition, this conduction was 'concealed' during sinus rhythm and right atrial pacing because the relatively prolonged right-to-left atrial conduction time allowed right atrial impulses to reach the His bundle via the atrioventricular node before they could do so through the atrioventricular nodal bypass tract. Case 2 had ectopic beats arising in a right-sided atrioventricular nodal bypass tract which did not conduct in either forward or retrograde directions, its presence being detected only when initiating impulses. However, it could not be determined whether this tract was an 'abnormal' atrio-His connection or a 'normal' transitional (atrio-atrioventricular nodal) tract. Though intracardiac studies complement body surface recordings, they should be interpreted with knowledge of their inherent limitations.