10 patients with an accessory atrioventricular pathway were studied by means of programmed electrical stimulation to test the feasibility of tachycardia prevention with an extrastimulus delivered after the tachycardia-initiating beat. Reciprocating atrioventricular tachycardia was reproducibly initiated from high right atrium in all 10 patients with a single extrastimulus. A second extrastimulus delivered to the same site resulted in prevention of tachycardia in all patients, when this extrastimulus was given within a "preventive zone" which started 10 ms after the effective atrial refractory period of the tachycardia initiating beat. The average width of the preventive zone was 50 ms in our patients, independent of the initial tachycardia interval or tachycardia cycle length. Tachycardia prevention was always successful with a single atrial extrastimulus, whereas termination of an ongoing tachycardia required at least two atrial or ventricular extrastimuli.