The symptoms arising from damage of the branches of the bundle of His have been studied for years by many investigators with the aim of ascertaining (a) if there exists a complete block (b) and if so which is the branch blocked. Most of the study has been devoted to the electrocardiographic changes, and though these changes are important for the diagnosis of bundle branch block, considerable difference of opinion has developed among investigators as to their value in determining the branch involved. Studying this problem experimentally and clinically, we have been able to find graphic signs of the ventricular asynchronism which is the fatal consequence of bundle branch block. Marked asynchronism in the contraction of the ventricles indicates with certainty that bundle branch block exists. Furthermore, it enables one to determine definitely which ventricle contracts first. VENTRICULAR ASYNCHRONISM IN EXPERIMENTAL BUNDLE BRANCH BLOCK Although slight asynchronism has been found