A new class I antiarrhythmic, flecainide, was investigated in 10 patients to assess short-term efficacy and safety. All patients were hospitalized for 3 days; no antiarrhythmics were given on days 1 and 3. On day 2 flecainide 1 mg/kg was given intravenously over 5 min. If the predrug arrhythmia(s) was not completely suppressed, additional boluses of 0.5 mg/kg were injected at 30- to 60-min intervals to a maximum of 5 mg/kg. Seven patients received 2 mg/kg, and 3 patients received 1 mg/kg. Before drug 5 patients had premature ventricular contractions (PVCs) (more than 4 per min); 2 patients had atrial fibrillation (AF) with PVCs; 1 patient had both PVCs and premature atrial contractions (PACs); and 2 patients had only PACs. One patient with PVCs failed to respond to flecainide; he was unresponsive to all available antiarrhythmic drugs. In the other 7 patients PVCs were suppressed for an average of 13 hr (range 6 to 24 hr), AF was not affected, and PACs were suppressed. Flecainide did not induce significant changes in P-R, QRS, or Q-T intervals. Side effects were negligible and included a tingling sensation and feeling of skin warmth for 15 min after drug in one patient.