We report an 18-year-old woman, who had undergone surgical repair of tetralogy of Fallot, and then underwent successful intracardiac recordings within the zone of slow conduction (ZSC) of the reentry circuit on the initiation of premature ventricular contractions (PVC) and reentrant ventricular tachycardia (VT), obtained with multielectrode basket catheter. PVC a) were caused by wavefronts of sinus cycles entering and exiting the ZSC, and b) were not repetitive because the wavefront of the PVC could not reenter the ZSC. Similarly, VT developed after delayed potentials of sinus cycles, and was sustained because the wavefront following the 1st cycle of VT was able to repetitively reenter and exit the ZSC. Thus, spontaneous changes in the conductivity of the ZSC, like an on-off switch, appeared to determine the onset and sustenance of reentrant VT.