Eleven patients with tachycardia-dependent, bradycardia-dependent, or "pseudobradycardia-dependent" bundle branch block (BBB) alternans were studied. This classification is based on the following criteria: 1) When alternans is initiated by a sudden acceleration in ventricular rate, or it appears with aberration of the second beat after a pause, the alternans is tachycardia-dependent and results from a 2:1 bidirectional block in the affected bundle branch. 2) When alternans begins with the aberrant complex terminating a pause it is bradycardia-dependent; such an alternans results from alternating bundle branch cycle lengths and refractoriness, possibly produced by alternating transseptal retrograde penetration of the affected bundle branch. 3) In cases referred to as "pseudobradycardia-dependent BBB" alternans, a change from alternans to persiscardia-dependent BBB" alternans, a change from alternans to persistent BBB occurs as the cycle lengthens; however, the disappearance of BBB with further increase of the cycle length proves the tachycardia-dependence of the conduction defect.