We studied the effect of drive train duration on human right bundle branch refractoriness. Seven patients with a QRS duration of 0.10 sec or less and no preexcitation had functional right bundle branch block induced reproducibly with premature atrial stimulation. Refractoriness of the right bundle branch was defined as the longest H1H2 interval resulting in right bundle branch block and was determined in each patient at a constant pacing cycle length but at five to seven different atrial drive train durations varying from four to 99 complexes. In all seven patients, right bundle branch refractoriness decreased with increasing drive train duration (mean 15 msec). One-third of this decrease (5.0 msec) occurred between drive train durations of four and eight complexes, and close to two-thirds (9.3 msec) occurred between drive train durations of eight and 64 complexes. Very little further decrease (0.7 msec) occurred between 64 and 99 complexes. We conclude that right bundle branch refractoriness shortens progressively as the preceding drive train duration increases. This phenomenon may in part explain the disappearance of functional right bundle branch block during supraventricular tachycardia after a variable number of complexes without a change in cycle length of tachycardia.