The authors report the case of a 70 year old woman with frequent attacks of supraventricular tachycardia resistant to antiarrhythmic therapy. The tachycardia was irregular with predominantly normal QRS complexes. Electrophysiological investigation showed dual conduction in the atrioventricular node and tachycardia was induced by atrial extrastimulus. However, reentrant tachycardia could not be induced, the refractory period of the slow pathway being much longer than that of the rapid pathway. The mechanism of the tachycardia was simultaneous conduction of the sinus rhythm through the two nodal conduction pathways. This was successfully treated by radiofrequency ablation of the slow pathway.