The various patterns resulting from stimulation through the catheter electrodes recording His bundle activity were evaluated in 30 patients using intracardiac electrograms from the right ventricular apex (RVA), posterosuperior wall of the left ventricle (LV), high right atrium (HRA) and left atrium (LA) in the vicinity of the coronary sinus. His bundle pacing was characterized by a QRS complex and stimulus (St)-V, St-RVA and St-LV intervals that equated the QRS configuration, H-V, H-RVA and H-LV intervals of sinus beats. Right septal pacing produced pattern of "complete" left bundle branch block (with normal electrical axis) associated with St-V intervals of 0 msec, and St-RVA and St-LV intervals of different duration from that of the H-RVA and H-LV intervals recorded during sinus rhythm. Fusion beats resulting from simultaneous activation of His bundle and right septal muscle were characterized by St-V intervals of 0 msec and St-RVA or St-LV intervals of similar duration to that of the H-RVA or H-LV intervals of sinus beats. Fusion QRS configuration depended on the type of ventricular complex present during sinus rhythm. Analysis of the retrograde atrial activation intervals permitted differentiation among impulse initiation at the low right atrium, His bundle or right septal muscle. Simultaneous recording of multiple atrial and ventricular electrograms has enhanced understanding of the complex patterns observed during attempted His bundle pacing in man.