Atrioventricular nodal alternating Wenckebach periods were defined as episodes of 2:1 atrioventricular block in which there was a gradual increase in transmission intervals of conducted beats ending in two or three consecutively blocked atrial impulses. This is one of the mechanisms whereby 2:1 atrioventricular block progresses into 3:1 or 4:1 atrioventricular block. Alternating Wenckebach periods appear during rapid atrial pac,ng (even in the absence of depressed atrioventricular nodal function), provided that the atria can be captured at a rate fast enough to allow for the occurrence of this phenomenon. Treatment of atrial flutter with digoxin and quinidine produces alternating Wenckebach's periods, with associated electrocardiographic changes specific for the type of drug given. In patients with "atrial tachycardia with atrioventricular block" due to digitalis intoxication or with primary disease of the conducting system or with acute myocardial infarction, there are coexisting severe arrhythmias and clinical symptoms requiring almost immediate pharmacologic or electrical therapy. We conclude that atrioventricular nodal alternating Wenckebach's periods are common and frequentyly transient and that they occur in a variety of clinical conditions, most of which are benign; however, contrary to what is commonly accepted, some episodes appear in clinical settings requiring prompt pharmacologic or electrical treatment.