In nine patients with left bundle-branch block (LBBB), normal QRS axis, and normal P-R interval, atrial, His bundle, and ventricular electrograms were recorded. The extrastimulus method was utilized to determine the relative refractoriness of the various components of the atrioventricular conducting system.The functional refractory period (FRP) of the A-V node was normal in all but one patient. In six patients, the effective refractory period (ERP) of the ventricular specialized conducting system (VSCS) was greater than the FRP of the A-V node. The differential refractoriness within the three components of the VSCS was determined. Before the ERP of the VSCS was reached, a leftward shift in the mean QRS axis was noted in all six patients, thus documenting participation of the left anterior division of the LBB in ventricular depolarization. We found no evidence that the left posterior division participated in ventricular activation and conclude that it is functionally silent in LBBB. Conduction abnormalities in the right bundle branch were demonstrated both by long H-V intervals and the fact that its ERP was prolonged.The increased refractoriness of all three divisions of the VSCS suggests that the conduction abnormalities producing evidence of LBBB on the surface are trifascicular in nature.