We analyzed routes of retrograde impulse propagation in the His-Purkinje system (HPS) in 24 patients with normal intraventricular conduction. Using intracardiac recordings from the His bundle and right bundle branch (RBB), predetermined paced ventricular cycle lengths were scanned with right ventricular extrastimuli (S2). Electrophysiologic findings suggested that retrograde activation of the His bundle after premature stimuli (H2) occurred via the left bundle branch (LBB) in 16 of 24 cases and via the RBB in three of 24 patients. The remaining patients demonstrated retrograde H2 activation via both RBB and LBB at different times during the scanning. In general, a linear relationship existed between retrograde conduction delays in the HPS (i.e., S2H2 intervals) and ventricular coupling intervals (S1S2) when retrograde H2 activation consistently occurred via the same bundle branch. However, we noted sudden and unexpected increases (30-90 msec) or decreases (10-55 msec) in S2H2 intervals in 11 of 24 patients; these changes occurred with and without concomitant changes in the retrograde His-right bundle (HRB) activation sequence. Retrograde H2 activation during right ventricular premature stimulation occurs exclusively through the LBB in a majority (67%) and through the RBB in a minority (12%) of cases. In some patients (21%), however, the route of retrograde H2 activation may vary and could occur via both the LBB and the RBB. Recordings from the HRB area should help to delineate the route of retrograde impulse propagation to the bundle of His.