Electrostimulation of the heart is of three-fold value for the treatment of tachyarrhythmias: 1. It proves the presence of an arrhythmogenic abnormality, 2. renders proper diagnosis of the type of tachycardia possible, and 3. forms the base for logical treatment. For therapeutic purposes temporary stimulation of many paroxysmal tachycardias is an alternative to DC-conversion. Initiation of arrhythmias in combination with acute drug testing helps in selecting efficient long-term treatment, especially in ventricular tachycardias. The fact that 50-100% of paroxysmal tachycardias can be interrupted by electrostimulation has rendered possible the development of antitachycardia pacemakers, a principle which may assume great importance in the future.