The differential diagnosis of tachycardias with a long RP interval is reviewed and the methods of treatment of these arrhythmias catheter ablation are described. Electrocardiographic and electrophysiologic criteria for the correct diagnosis of atrial tachycardia, circus movement tachycardia using retrogradely an accessory pathway with decremental conduction properties and the uncommon form of atrioventricular nodal reentrant tachycardia are discussed. First results of our institution of radiofrequency catheter ablation of atrial tachycardias and circus movement tachycardias using retrogradely an accessory pathway with decremental conduction properties are presented. We concluded that both electrocardiographic and electrophysiologic criteria give a better understanding of the mechanism and arrhythmic site which are important markers for a safe and successful ablation procedure.