Four patients are described with different degrees of conduction disturbance within the His bundle. In one patient with a Mobitz type II atrioventricular (A-V) block with normal QRS complexes the blocked atrial beats were followed by a His potential. Since the QRS complexes of the conducted beats were completely normal, the site of the block was thought to be in the distal part of the His bundle. In the other three patients with a 2:1 A-V block, a nearly complete A-V block, and a complete A-V block, two distinct His potentials could be discerned, one (H) following each atrial beat, the other (H′) preceding each ventricular activation. In the patient with 2:1 A-V block a Wenckebach phenomenon within the His bundle could be produced at certain atrial driving rates. Impulse conduction through the A-V node was normal in all four cases, as could be concluded from the effect of increased atrial driving rate and of accurately timed atrial premature beats. The site of the block could not be predicted from the conventional electrocardiogram.