The effect of flecainide acetate, a class 1c antiarrhythmic agent, was examined in 15 patients with recurrent ventricular tachycardia. Intravenous flecainide was administered in a dose of 2 mg/kg at the time of intracardiac stimulation and recording studies. Oral flecainide was given to 10/15 patients and retesting was undertaken using an indwelling electrode. Intravenous flecainide terminated sustained stable tachycardia in 8/11 patients and prevented reinitiation of tachycardia in 5/10 patients. Oral therapy prevented induction of tachycardias in only 2/10 patients. Five patients had non-sustained tachycardia and three had slower sustained tachycardia. "New" non-clinical tachycardias could be induced in six patients after flecainide but five of these had had more than one type of induced tachycardia. Four of 10 patients remained free of tachycardias during follow-up. Withdrawal of oral treatment was necessary in three patients, one of whom had severe proarrhythmic effects. Two patients required additional antiarrhythmic therapy. Long-term suppression could not be predicted from the results of oral therapy, but testing after intravenous drug seemed to be a more useful prognostic indicator. In summary, intravenous flecainide is effective for slowing and termination of stable ventricular tachycardia. Oral therapy is also effective but caution should be exerted in patients with multimorphic tachycardias.