To demonstrate the occurrence of concealed conduction in anomalous atrioventricular (AV) bypass tracts, 11 patients were selected for study. Two had a right-sided and nine had a left-sided bypass tract. Electrode catheters were placed in the right atrium, coronary sinus, AV junction and right ventricle. After every eighth atrial or ventricular driving beat (A1 or V1) at a constant cycle length, two successive atrial or ventricular premature beats (A2 and A3 or V2 and V3) were delivered. The A1A2 or V1V2 interval was fixed at 30 ms greater than the effective refractory period of the atrium or right ventricle, but less than the effective refractory period of the bypass tract in the anterograde or retrograde direction. This allows A2 or V2 to capture the atrium or ventricle, but not conduct in the bypass tract. The A3 or V3 was delivered from late diastole with a progressively shorter A2A3 or V2V3 interval until atrial or ventricular refractoriness was encountered. In the anterograde direction, the presence of A2 prevented A3 conduction in the bypass tract despite A1A3 intervals being longer than the anterograde effective refractory period of the bypass tract in 8 of the 11 patients. In the retrograde direction, the presence of V2 prevented V3 conduction in the bypass tract despite V1V3 intervals being longer than the retrograde effective refractory period of the bypass tract in 3 of the 11 patients. Thus, using the technique of programmed electrical stimulation, concealed conduction in anomalous AV bypass tracts can be demonstrated in both anterograde and retrograde directions.