The usefulness of the response to single and double ventricular premature complexes (VPCs) introduced during reciprocating tachycardia (RT) in predicting the location of a left free wall accessory pathway was studied in 55 patients with the Wolff-Parkinson-White syndrome. One VPC introduced from the right ventricle into narrow QRS RT when the His bundle was refractory resulted in retrograde atrial preexcitation in 25 of 55 (45%) patients, while 30 (55%) showed no preexcitation. Double VPCs produced retrograde atrial preexcitation in 9 of 26 patients not responding to a single VPC. No difference in RT cycle length, AH, HV or ventriculoatrial intervals was found between those patients who did or did not show retrograde atrial preexcitation. The response to single and double VPCs during RT was related to the location of the AP. The average distance of the AP from the crux determined by intraoperative epicardial mapping in the 41 patients who underwent surgery was 2.7 +/- 0.7 mapping units (left posterolateral region) in patients showing retrograde atrial preexcitation with a single VPC, 3.6 +/- 0.7 units (at the lateral left ventricular margin) in those responding to double VPCs and 4.3 +/- 0.8 units (beyond the LV margin) in those showing no response. Left bundle--branch block (LBBB) aberrancy during RT resulted in an average 60 +/- 14 ms prolongation of the ventriculoatrial interval in 40 patients, including 5 in whom LBBB was seen only after procainamide infusion. VPCs introduced into LBBB RT resulted in significant retrograde atrial preexcitation in 6 additional patients in whom no response during normal QRS RT was observed.(ABSTRACT TRUNCATED AT 250 WORDS)