Endocavitary catheter ablation by radiofrequency energy applied on the slow pathway is an effective method of treatment of nodal reentrant tachycardias. The aim of this report was to determine the criteria predictive of success during radiofrequency ablation of the slow pathway guided by the presence of slow potentials. Thirty-five patients (21 women, 14 men, mean age 44 +/- 14 years) with frequent attacks of junctional tachycardia were studied. After confirmation of the diagnosis by electrophysiological investigation, radiofrequency energy was delivered at a site characterised by the presence of slow potentials between the atrial (A) and ventricular (V) potentials. The criteria investigated at each site were: before application: A/V ratio; amplitude of A and V: maximum A/minimum A ratio; amplitude and duration of the A potential; during ablation: radiological stability of the catheter position and occurrence of a junctional rhythm. All 35 patients had successful procedures with no inducible tachycardia at the end of the procedure. The slow pathway was destroyed in 20 cases (57%) with no complication of atrioventricular block. The duration of the A potential was longer in the successful cases (56 +/- 16 vs 48 +/- 14 ms; p = 0.04). The appearance of junctional rhythm and catheter stability during the procedure were predictive of success (79% vs 48%; p = 0.02; 74% vs 43%; p = 0.01). The authors concluded that an ablation site with a long duration A potential and a slow potential is a good target. In addition, ablation should be started when the catheter is radiologically stable and should not be interrupted in the absence of a junctional rhythm.