Used orally, in double-blind cross-over controlled trials conducted on 73 patients, disopyramide, a newly developed antiarrhythmic agent, was found to be more effective than an inert placebo (lactose), with a statistically significant difference, and to be as effective as quinidine sulfate in suppressing ventricular and supraventricular premature beats. In double-blind studies with 42 patients, it was also found to be as effective as quinidine sulfate in preventing the recurrence of atrial fibrillation after successful cardioversion. In non-blind studies attacks of paroxysmal supraventricular and ventricular tachycardia and transient atrial fibrillation were effectively prevented in the respective arrhythmias in more than 70% of the cases.