Propafenon, at an initial oral dose of 900 mg, proved an effective anti-arrhythmic drug in 34 patients with frequent ventricular and(or) supraventricular extrasystoles at rest. Long-term ECG monitoring of seven patients revealed that this effect lasted about eight hours. It can be maintained by an oral dose of 300 mg propafenon at eight-hourly intervals, as demonstrated in 21 further patients. Rapid anti-arrhythmic action was achieved in six acute cases with 1.0-1.5 mg propafenon per kg body weight by quick infusion, and lasted for one to one-and-a-half hours. In such cases the drug should, therefore, be given synchronously by the oral route. Side effects related to the gastrointestinal tract, sinus automatism as well as atrio-ventricular and intraventricular conduction, but in no case were very serious.