The effects of the antiarrhythmia agents amodiaquin and quinidine on conduction times through the atrium and the specialized conducting tissue to the ventricular myocardium have been determined from records obtained with needle electrodes suitably placed in isolated blood perfused dog hearts. Quinidine slowed conduction through each portion of the total pathway when very low concentrations were infused but had a preferential depressant effect on atrial conduction at slightly higher concentrations. Amodiaquin slowed conduction in the atrioventricular node to a greater extent than in the other parts of the common pathway at all concentrations tested. The major effect of each drug on extra-atrial conduction occurred at the atrioventricular node and at the Purkinje–myocardial junction. Amodiaquin did not induce bigeminal rhythm in the isolated heart.