Transesophageal electrophysiologic studies were performed in 58 infants (age ≤ 1 year, median 10 days) with electrocardiographically documented orthodromic reciprocating tachycardia (ORT). The aim was to evaluate the occurrence, type and electrophysiologic effects of bundle branch block (BBB) during ORT onset. Of the 58 infants, 25 (43%) had BBB with pacing-induced tachycardia onset. BBB was initiated by single or double premature atrial extrastimuli and by burst atrial pacing; 4 infants also demonstrated BBB with spontaneous ORT onset during transesophageal study. Two of 25 infants had BBB only after intravenous procainamide. Comparison of the 25 infants exhibiting BBB at ORT onset with the 33 infants not demonstrating BBB revealed that age was not statistically different in the 2 groups, but that severity of illness (based on a 1 to 3 scale) was greater (p < 0.05) and normal QRS ORT cycle length was shorter (p < 0.02) in the infants with BBB. Of the 25 infants with BBB at ORT onset, 17 had left BBB, 3 had right BBB and 5 had both left and right BBB. Ventriculoatrial interval or cycle length increases during ORT with BBB in 16 of 25 (64%) infants suggested left free wall-accessory atrioventricular connections. These data show that in infants with ORT undergoing transesophageal study: (1) BBB during pacing-induced ORT onset is common; (2) isolated left or right BBB occurs more often than both, and left BBB is more common than right BBB; (3) BBB-induced changes in cycle length or ventriculoatrial interval permit localization of accessory atrioventricular connection site in many infants; and (4) when BBB changes permitted accessory atrioventricular connection location, left-sided connections were present.