Bepridil at cumulative doses between 1.25 and 8.75 mg/kg and quinidine between 2.5 and 17.5 mg/kg given in conscious dogs with chronic atrioventricular block and implanted atrial pacing electrodes, dose-relatedly lengthened atrial effective refractory period (AERP), as reflected by the decrease in maximal atrial frequency determined by pacing. 2. Diltiazem shortened AERP at 0.25 mg/kg and lengthened it at 1.75 mg/kg, but both effects were very slight. 3. Verapamil between 0.06 and 0.435 mg/kg did not alter AERP at all. 4. Except for diltiazem at 0.75 and 1.75 mg/kg and bepridil at 8.75 mg/kg, each dose of each drug increased atrial rate. Each drug produced an increase in ventricular rate and a short-lasting lowering in mean blood pressure. 5. Thus, these results indicate that bepridil exhibits more marked antiarrhythmic potentialities than quinidine and that the atrial and ventricular tachycardic effects observed are mainly baroreceptor reflex effects.
