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Notes: Slow Pathway Ablation. Introduction: The relationship between temperature at the electrode-tissue interface and the loss of AV and ventriculoatrial (VA) conduction is not established, and the optimal target temperature for the slow pathway approach to radiofrequency ablation of AV nodal reentrant tachycardia (AVNRT) is unknown. Therefore, the purpose of this study was to compare target temperatures of 48°C and 60°C during the slow pathway approach to ablation of AVNRT. Methods and Results: The study included 138 patients undergoing ablation for AVNRT. Patients undergoing slow pathway ablation using closed-loop temperature monitoring were randomly assigned to a target temperature of either 48°C or 60°C. The primary success rates were 76% in the patients assigned to 48°C and 100% in the patients assigned to 60°C (P 〈 0.01). The ablation procedure duration (33 ± 31 min vs 26 ± 28 min; P = 0.2), fluoroscopic time (25 ± 15 min vs 24 ± 16 min; P = 0.5), and mean number of applications (9.3 ± 6.5 vs 7.8 ± 8.1; P = 0.3) were similar in patients assigned to 48° and 60°C, respectively. The mean temperature (46.1°± 24.8°C vs 48.7°± 3.2°C; P 〈 0.01), the temperature associated with junctional ectopy (48.1°± 2.0°C vs 53.5°± 3.5°C, P 〈 0.0001), and the frequency of VA block during junctional ectopy (24.6% vs 37.2%; P 〈 0.0001) were less in the patients assigned to 48°C compared to 60°C. The frequency of transient or permanent AV block was similar in each group (2.8% vs 3.6%; P = 0.2). In the 60°C group, only 12% of applications achieved an electrode temperature of 60°C. During follow-up of 9.9 ± 4.2 months, there was one recurrence of AVNRT in the 48°C group and none in the 60°C group. Conclusions: Compared to 48°C, a target temperature of 60°C during radiofrequency slow pathway ablation is associated with a higher primary success rate and a higher incidence of VA block during junctional ectopy induced by the radiofrequency energy. AV block is not more common with the higher target temperature, but only if VA conduction is aggressively monitored during applications of radiofrequency energy.