Successful transvenous catheter ablation of accessory pathway (AP) in a patient (pt) with the permanent form of junctional reciprocating tachycardia is reported. A concealed AP with long conduction time comprised the retrograde limb of tachycardia circuit. The atrial end of the AP was near the coronary sinus (CS) orifice. Catheter ablative technique was performed after assessment of its feasibility and safety by using a canine model. During an electrophysiologic study CS mapping was obtained; the earliest retrograde activity was recorded 1-2 cm inside the orifice of the CS. Two unipolar shocks of 120 J were delivered on the sites exhibiting the earliest retrograde atrial activity. After the procedure, the AP was no longer conducting in both anterograde and retrograde direction. During a follow-up period of 13 months, the pt remained free of tachycardia on no antiarrhythmic medication.