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Background: Atrial tachycardia (AT) can be treated by medical or electrical cardiover- sion but the recurrence rate is high. Three-dimensional electro-anatomical mapping, recently described in horses, might be used to map AT to identify a focal source or reentry mechanism and to guide treatment by radiofrequency ablation. Objectives: To describe the feasibility of 3D electro-anatomical mapping and radi- ofrequency catheter ablation to characterize and treat sustained AT in horses. Animals: Nine horses with sustained AT. Methods: Records from horses with sustained AT referred for radiofrequency abla- tion at Ghent University were reviewed. Results: The AT was drug resistant in 4 out of 9 horses. In 8 out of 9 horses, AT origi- nated from a localized macro-reentrant circuit (n = 5) or a focal source (n = 3) located at the transition between the right atrium and the caudal vena cava. In these 8 horses, local radiofrequency catheter ablation resulted in the termination of AT. At follow- up, 6 out of 8 horses remained free of recurrence. Conclusions and Clinical Importance: Differentiation between focal and macro- reentrant AT in horses is possible using 3D electro-anatomical mapping. In this study, the source of right atrial AT in horses was safely treated by radiofrequency catheter ablation.