The participation of intraventricular conduction defects in the AV delays and blocks has been investigated by His bundle electrogram recording in 239 patients with different degrees of AV blocks and QRS enlargement (greater than 0.12 sec). In absence of PR prolongation, the His bundle electrogram can demonstrate intraventricular conduction delay (HV superior to 55 msec) with an increasing frequency in right bundle branch block and right bundle branch with left axis deviation, left bundle branch block and right bundle block with right axis deviation. In cases of first-degree AV block (PR greater than 0.20 sec) delay within the His bundle is present in 20% of the cases and HV prolongation, isolated or associated with an upper conduction defect is demonstrated in 66% of the cases. Second-degree AV block with QRS enlargement in the conducted beats is due to a subnodal lesions of the conducting tissue in 80% of the cases. Wenckebach phenomenon and bundle branch block is as frequent above as below the site of His bundle electrogram recording. Möbitz II block has always an infranodal localization. Third-degree AV block with wide QRS complexes is the consequence of a lesion within the His bundle in 11% and of a complete bilateral bundle branch block in 78% of the cases. Exploration of the AV conduction in acute myocardial infarction with AV block confirms the usual bilateral bundle branch lesion in anterior myocardial necrosis and the AH localization of the AV block in posterior myocardial infarction even in presence of enlarged QRS complexes. Unidirectional block occurs in 19 of the 82 cases of complete anterograde bilateral bundle branch block, with retrograde conduction to the atria in 11 and concealed retrograde conduction in 8 cases.