A patient with the Wolff-Parkinson-White syndrome and recurrent bouts of paroxysmal supraventricular tachycardia under-went electrophysiologic studies. These studies revealed evidence of dual atrioventricular nodal pathways and a septal accessory pathway. The tachycardia circuit involved anterograde conduction over a slow atrioventricular nodal pathway and retrograde conduction over the accessory pathway. Spontaneous tachycardia termination was frequently observed, was almost always abrupt, and was associated with a beat-to-beat decrease in the A-H interval. In this patient, the mechanism for spontaneous tachycardia termination appeared to be an abrupt shift in anterograde conduction from the slow to the fast pathway, advancing the tachycardia so that the block occurred in the accessory pathway (or atrium). To our knowledge, this mechanism of spontaneous tachycardia termination has not been described previously. Use of agents to facilitate fast atrioventricular nodal conduction (i.e., atropine) may be effective in tachycardia termination for these patients.