Five patients with apparent adverse cardiovascular effects of disopyramide are reveiwed. Attention is drawn to the following problems. (1) A vagolytic effect may produce a sinus tachycardia with wide QRS complexes due to aberrant conduction or intraventricular block superficially resembling a ventricular tachycardia, or may allow increased transmission of an atrial tachycardia or atrial flutter to the ventricles by improving atrio-ventricular conduction. (2) Although the vagolytic effect is helpful in increasing sinus rate in patients with sinus node disease, disopyramide may lead to bradycardia and asytolic cardiac arrest, and should not be used without a demand pacemaker. (3) Dangerous ventricular arrhythmias may be provoked in susceptible subjects, as with quinidine. (4) Rapid intravenous injection may produce transient toxic effects before the drug is distributed. The rate of injection as a loading dose for prophylaxis should be slower (2 mg/kg in 15 min) than for the urgent conversion of a resistent tachycardia (2 mg/kg in 5 min). Although disopyramide seems less toxic than quinidine, caution is advised, as over-enthusiastic application of disopyramide, particularly with rapid intravenous injection, might tend to bring a useful new agent into disrepute.