The purpose of this study was to correlate changes in electrical impedance with the electrode-tissue interface temperature and to characterize the associated events occurring at the catheter tip electrode. In a canine model, lesions were created in vitro (n = 49) and in vivo (n = 31) and radiofrequency power settings were varied. Electrode-tissue interface temperature, delivered current, and voltage were recorded, and impedance was calculated. A sudden rise in electrical impedance was seen in only two of 17 ablations in vitro and in one of 16 ablations in vivo with a peak electrode-tissue interface temperature of less than 100 degrees C compared with 29 of 32 ablations in vitro (p = 0.0001) and 12 of 15 ablations in vivo with a temperature of more than 100 degrees C (p = 0.0001). This phenomenon was associated with the observation of boiling and popping at the tip in in vitro preparations and tissue avulsion and thrombus formation on the catheter tip in in vivo studies. The lesion size was directly proportional to the peak temperature for all ablations but not to the peak power, current, or voltage during radiofrequency catheter ablation in the heart. Maintaining electrode-tissue interface temperature at less than 100 degrees C during radiofrequency catheter ablation in the heart may avoid the complications associated with the sudden rise in electrical impedance.