Although uncommon, incessant supraventricular tachycardia (the daily presence of supraventricular tachycardia for more than 50% of the day) is a major therapeutic problem. Using programmed electrical stimulation of the heart, long-term electrocardiographic monitoring and exercise testing, the effect of intravenous and oral encainide for termination and prevention of incessant supraventricular tachycardia was assessed in 11 patients (aged 25 to 58 years). All patients had received 3 to 12 drugs (mean 6) without control of their arrhythmia. Eight patients suffered from incessant supraventricular tachycardia using an accessory pathway in retrograde direction (three with overt Wolff-Parkinson-White syndrome, one with a concealed accessory atrioventricular [AV] pathway of the fast type, three with a concealed accessory AV pathway of the slow type and one with a nodo-ventricular accessory pathway). Three patients had incessant atrial tachycardia, one of whom also had the Wolff-Parkinson-White syndrome. Intravenous encainide (1.5 mg/kg in 15 minutes) terminated incessant supraventricular tachycardia in seven of nine patients. In four of nine patients, supraventricular tachycardia could thereafter still be reinitiated by pacing. Oral encainide (100 to 325 mg/day, mean 180) completely suppressed the incessant supraventricular tachycardia in eight patients in a follow-up period of 5 to 20 months (mean 11). In two patients, episodes of tachycardia were markedly reduced with the administration of encainide in combination with sotalol (one patient) and amiodarone (one patient). Encainide failed to control incessant tachycardia in one patient. Mild central nervous system side effects developed in two patients, but both could continue taking oral encainide. Encainide proved to be a very useful agent to control incessant supraventricular tachycardia resistant to other antiarrhythmic agents.