RF current delivery may cause acute and chronic dysfunction of previously implanted pacemakers. The aim of this study was to assess prospectively the effects of RF energy on Thera I and Kappa pacemakers in 70 consecutive patients (mean age 70 +/- 11 years, mean left ventricular ejection fraction 48 +/- 15%) who underwent RF ablation of the AV junction for antiarrhythmic drug refractory atrial fibrillation (permanent in 42 patients, paroxysmal in 28). These pacing systems incorporate protection elements to avoid electromagnetic interference. The pacemakers (Thera DR 7960 I in 20 patients, Thera SR 8960 I in 30, Kappa DR 600-601 in 8, Kappa SR 700-701 in 12) were implanted prior to RF ablation in a single session procedure and were transiently programmed to VVI mode at a rate of 30 beats/min. Capsure SP and Z unibipolar leads were used. During RF application there was continuous monitoring of three ECG leads, endocavitary electrograms, and event markers. Complete AV block was achieved in all cases after 3.6 +/- 2.9 RF pulses and 100 +/- 75 seconds of RF energy delivery. The mean time of pacemaker implantation and RF ablation was 60 +/- 20 minutes. Transient or permanent pacemaker dysfunction including under/oversensing, reversion to a "noise-mode" pacing, pacing inhibition, reprogramming, or recycling were not observed. Leads impedance, sensing, and pacing thresholds remained in the normal range in the acute and long-term phase (average follow-up 18 +/- 12 months). In conclusion, Thera I and Kappa pacemakers exhibit excellent protection against interference produced by RF current. The functional integrity of the pacemakers and Capsure leads was observed in the acute and chronic phases. Thus, the implantation of these pacing systems prior to RF ablation of the AV junction can be recommended.