The adverse interactions of permanent pacemakers and automatic implantable cardioverter defibrillators (AICD) were studied in nine patients in whom both devices were implanted. Both unipolar and bipolar pacemakers were evaluated. The permanent pacemakers were also used to do noninvasive electrophysiological studies and to induce ventricular fibrillation. Undersensing of ventricular fibrillation by the permanent pacemakers caused inappropriate pacemaker stimuli, which caused undersensing of ventricular fibrillation by the AICD in three of four patients with unipolar pacemakers. After an AICD discharge, pacemaker noncapture was seen in eight of 22 episodes for an average 4.9 seconds and inability to sense was seen in 11 of 20 episodes for an average 9.0 seconds. Counting of pacemaker stimuli and QRS by the AICD caused inappropriate discharges. Noninvasive electrophysiological testing by the pacemakers correlated with invasive testing. Furthermore, induction of ventricular fibrillation was successful in four of five patients attempted, though requiring long bursts at high outputs at the shortest cycle lengths obtainable by these pacemakers. Operation of the AICD and permanent pacemakers must be clearly understood to avoid adverse interactions of these devices.