Background Diastolic potentials (DP) are reported to be recorded in intracardiac electrograms during verapamil-sensitive ventricular tachycardia (VT) in which QRS complexes show complete right bundle branch block with a superior axis. The purpose of this study was to ascertain whether the DP recorded in the endocardial mapping during VT reflects the activation of the VT circuit. Methods and Results The study group consisted of 16 men and 2 women. The earliest activation site (EA site) was determined and the DP was recorded in the endocardial mapping during VT. We evaluated the response of the cycle length of VT, the interval between the ventricular activation and the DP (V-DP), and the interval between the DP and the ventricular activation (DP-V) to intravenous verapamil. Radiofrequency current was delivered to the EA site, the site where the DP was recorded, and the site where the DP and the Purkinje fiber potential of the left bundle branch (LB) were simultaneously recorded. In 15 patients, the DP was recorded in the wide posterior fascicle region of the LB. After verapamil, the cycle length of VT, the V-DP, and the DP-V were prolonged from 365 ± 53 to 490 ± 65, 315 ± 30 to 368 ± 30, and 50 ± 27 to 123 ± 36 ms, respectively, in 6 patients. The LB was recorded in all patients and the DP was recorded preceding the LB in 12 patients. VT was successfully ablated at the site where the DP and the LB were simultaneously recorded in all these patients. Ablation at the other sites failed. Conclusions Radiofrequency ablation at the site where the DP was simultaneously recorded preceding the LB completely abolished the verapamil-sensitive VT. The DP recorded with the LB simultaneously might reflect the slow conduction zone activity of the reentry circuit located within the Purkinje fiber network. (Am Heart J 1999;138:560-6.)