In order to correlate the antiarrhythmic and electrophysiological effects of disopyramide phosphate during acute myocardial ischemia, we performed experiments in 17 mongrel dogs. Refractory periods obtained by the extrastimulus method and conduction times recorded from local electrograms were determined in potentially ischemic and nonischemic areas prior to, after left anterior descending coronary occlusion, and following intravenous administration of disopyramide phosphate 3 mg./Kg. Control refractory periods were similar in both nonischemic and ischemic areas. Following coronary ligation, a disparity of refractoriness of 28 msec. was induced between these two areas. After disopyramide administration, this disparity was reduced from 28 msec. to 5 msec. (p less than 0.001) after 5 to 15 minutes, and to 15 msec. (p less than 0.01) after 15 to 30 minutes. Coronary ligation prolonged conduction times by 8 msec. (p less than 0.005) in ischemic areas and disopyramide further prolonged conduction in these areas by an additional 9 msec. (p less than 0.001). A minimal and transient prolongation of conduction was present in nonischemic areas. We conclude that the differential effects exerted by disopyramide phosphate in ischemic areas may explain its suppressant action of arrhythmias of ventricular origin.