In a patient with atrial tachycardia with a rate of 200 per minute, the A-V conduction ratio was at times 2:1, but often it was 4:3 or 3:2 with progressive P-R interval prolongation (Wenckebach mechanism, an expression of presumable A-V nodal block). In each episode of 4:3 conduction, the first QRS complex was narrow, and the 2 ensuing beats were wide due to aberrant conduction. Aberration did not occur with a constant configuration, but in consecutive episodes of 4:3 conduction ratio there was a regular alternation of left bundle branch block and right bundle branch block. The pattern was explained by concealed retrograde conduction into the anterogradely blocked bundle branch. This caused 2 distinct effects: (1) shifting "to the right" of the refractory period of the affected bundle branch, resulting in maintenance of aberration with the same configuration, if consecutive atrial impulses were conducted to the ventricles, and (2) shortening of the effective cycle of the affected bundle branch, resulting in aberration due to block of the controlateral bundle branch, whenever a pause occasioned by a nonconducted atrial impulse was followed by restoration of 1:1 conduction for 2 or more consecutive beats.