A case of bilateral bundle branch block is described with the following features: -- 2nd degree infranodal AV block due to bilateral, intermittently synchronous "phase 3" bundle branch block; -- occasional AV conduction failure due to concealed His bundle extrasystoles; -- right bundle branch block pattern due to retrograde activation from the left bundle branch; -- QRS complexes having normal duration due to uniform slow conduction in both bundle branches; -- Wenckebach phenomenon in the right bundle branch; -- concealed conduction in both branches. The above interpretations are based on the use of His bundle recordings, results of right atrial stimulation and of pharmacological testing. Determinations of the lengths of all H--H intervals, whether H was followed by a ventricular response or blocked, permitted insight into the mechanism of 2nd degree AV block with varying (right and left) bundle branch block.