Restoration of the sinus rhythm in atrial fibrillation and flutter can be achieved by cardioversion, using an electric discharge, or by medicamentous treatment. Medicamentous treatment is based above all on a combination of antiarrhythmic drugs. By the concurrent administration of quinidine, verapamil and digoxin restoration of the sinus rhythm is achieved in 80% patients, on average after 37 hours at plasma quinidine levels of 2.57 +/- 1.4 (SD) micrograms/ml and digoxin levels of 1.90 +/- 1.3 (SD) nmol/l. Restoration of the sinus rhythm in atrial flutter calls for higher quinidine and digoxin levels than in atrial fibrillation (p less than 0.01). Prolonged persistence of the sinus rhythm during treatment with maintenance doses of quinidine, verapamil and digoxin is not satisfactory so far and after 12 months the sinus rhythm persists only in 30% of the patients where the rhythm was originally restored.