Concealed intraventricular conduction is defined and the following classification of the manifestations of concealed conduction into the bundle branch system is proposed. 1. Trans-septal retrograde concealed intraventricular conduction responsible for (a) perpetuation of functional bundle branch block initiated by a premature supraventriculra impluse; (b) alternation of aberrant ventricular conduction in supraventricular bigeminy; (c) normalization of intraventricular conduction with acceleration or rate in bradycardia-dependent bundle branch block, and (d) prevention of the manifestation of Wenchbach periods of conduction in a bundle branch or fascicle. 2. Antegrade concealed intraventricular conduction responsible for (a) prevention of expected aberrant ventricular conduction when a short cycle follows a long one, and (b) exceptions to the "rule of bigeminy". 3. Retrograde concealed intraventricular conduction of a ventricular escape in association with unidirectional bundle branch or fasciular block responsible for (a) resumption of AV conduction in "paroxysmal AV block" with bundle branch block, and (b) facilitation (due to supernormality) of conduction in type II AV block due to bilateral bundle branch block. 4. Concealed intraventricular conduction of a premature ventricular impulse responsible for (a) initiation or termination of a re-entrant ventricular tachycardia; (b) resetting of an idioventricular pacemaker, and (c) pseudo-intraventricular or pseudo-AV block.