Fifty-one consecutive patients with atrial tachycardia [atrial fibrillation (FA) 37, atrial flutter (Fl) 10, tachycardia (TSA) 4] received a single 2 mg/kg dose of flecainide injected intravenously over 5 to 10 minutes. Patients over 75 years of age or presenting with heart failure or disorders of conduction were excluded from the study. Sinus rhythm was restored during the injection or within the following 5 minutes in 15 patients (FA 12, Fl 1, TSA 2). The most frequent side-effects were visual disorders (5 cases) and moderate prolongation of QRS (all cases). Five patients had more pronounced disorders of conduction, but none developed heart failure. Reduction was obtained in 12 of the 18 patients whose arrhythmia had lasted for less than 8 days, against only 3 of the patients whose arrhythmia was more than 8 days old. Drug-induced reduction always failed in patients with left atrial dilatation (greater than 45 mm at echocardiography). An increase in the ventricular rate of unreduced flutter was observed in only one patient. Plasma flecainide levels were high (1.5 and 5.5 mg/l) at the end of the injection, but they fell to the therapeutic range within less than 10 minutes. Flecainide therefore appears to be effective in reducing recent atrial fibrillation on a non-dilated atrium. In cases with disorders of conduction, altered ventricular function or flutter, the therapeutic risk is such that another method of conversion should be used.