Flecainide acetate, 2 mg/kg body weight, given intravenously at 10 mg/min was administered to 128 (74 male and 54 female) patients whose ages ranged from 11 to 86 years (mean 44). All patients had supraventricular tachycardias (SVT) that developed spontaneously or were induced during electrophysiologic study. There were 26 patients with atrial flutter, 34 with atrial fibrillation, 7 with ectopic atrial tachycardia, 41 with atrioventricular (AV) reentrant tachycardia and 40 with AV nodal reentrant tachycardia. Twenty patients had more than 1 variety of SVT. Flecainide was administered during SVT to 9 patients with atrial flutter, 11 with atrial fibrillation, 7 with atrial tachycardia, 38 with AV reentrant tachycardia and 34 with AV nodal reentrant tachycardia. In the remaining 31 patients with inducible SVT at electrophysiologic study, flecainide was administered during sinus rhythm. Reinitiation of SVT was attempted in these patients after completion of flecainide administration. Flecainide successfully terminated atrial flutter in 2 patients (22%), atrial fibrillation in 9 (82%), atrial tachycardia in 5 (71%), AV reentrant tachycardia in 32 (84%) and AV nodal reentrant tachycardia in 30 (88%). Reinitiation of SVT was possible in 10 of 26 patients with atrial flutter (38%), 5 of 34 with atrial fibrillation (15%), 3 of 7 with atrial tachycardia (43%), 14 of 41 with AV reentrant tachycardia (34%) and 11 of 40 with AV nodal reentrant tachycardia (27%). In patients with AV reentrant tachycardia and AV nodal reentrant tachycardia, reinitiation occurred when retrograde anomalous pathway refractoriness was not significantly prolonged by intravenous flecainide.(ABSTRACT TRUNCATED AT 250 WORDS)