Disopyramide phosphate was administered intravenously to 57 patients with 60 episodes of arrhythmia (21 supraventricular and 39 ventricular) as a 2 mg/kg bolus. Conversion to sinus rhythm was achieved in three (38 percent) of eight patients with atrial flutter, two (20 percent) of ten patients with atrial fibrillation, one (33 percent) of three patients with paroxysmal atrial tachycardia, and two (50 percent) of four patients with sustained ventricular tachycardia. In nine (75 percent) of 12 patients with nonsustained ventricular tachycardia, suppression of the arrhythmia was accomplished following the intravenous bolus of disopyramide. In 18 (78 percent) of 23 patients with frequent ventricular premature contractions, greater than 50 percent suppression of the ventricular premature contractions was achieved. These effects were satisfactorily maintained in six (86 percent) of seven patients with nonsustained ventricular tachycardia and in 14 (88 percent) of 16 patients with frequent ventricular premature contractions in whom therapy with disopyramide phosphate was continued as a 20 mg/hour intravenous drip infusion for up to 24 hours. Side effects were observed in only eight patients (14 percent) and were primarily anticholinergic in nature. Transient hypotension, not necessitating treatment with pressor agents, was observed in three patients (5 percent), in two of whom discontinuance of therapy with disopyramide was deemed necessary. Intravenous therapy with disopyramide in the dosage regimen employed appears to be moderately effective against supraventricular arrhythmia and particularly effective against ventricular arrhythmia with minimal toxicity. It appears to be a suitable alternative to intravenous therapy with lidocaine and has the additional advantage of availability for oral administration.