Intravenous flecainide (1.5 mg/kg) was given to 30 patients with acute supraventricular tachyarrhythmias: atrial fibrillation (12 patients), atrial flutter (7 patients), and reciprocating tachycardia due to reentry through an accessory pathway (5 patients). The total rate of conversion to sinus rhythm was 18 out of 30 patients (60%), 11 out of 11 patients (100%) with reciprocating tachycardia and 7 out of 19 patients (37%) with atrial flutter or fibrillation. In those patients not converted to sinus rhythm, the mean ventricular rate significantly (P<0.1) slowed from 140 ± 15 beats/min with slowing of ventricular rate below 100 beats/min in 2 patients. Tolerable side effects occurred in 5 patients (17%) and included transient moderate hypotension (1 patient), blurred vision (1 patient), and transient conduction defects (3 patients). We conclude that intravenous flecainide is a safe and effective antiarrhythmic agent for the termination of acute reciprocating tachycardias and moderately useful for the management of atrial flutter or fibrillation