This paper describes a new technique for terminating attacks of supraventricular tachycardia in a patient with Wolff-Parkinson-White syndrome by using a demand pacemaker. The circuitry of this demand pacemaker generator is designed so that a magnet held near the generator converts the unit from demand to fixed mode at a pre-set discharge rate of 72/ min. During an attack of tachyeardia, a magnet held near the generator pocket activates a fixed rate discharge and produces competitive pacing. Retrograde atrial depolarization occurs with resultant reversion to sinus rhythm.