In this communication are presented the results of a detailed study, by means of the electrocardiograph, of six individuals exhibiting the change from a normal heart rhythm to that of auricular fibrillation. Although the disturbance of cardiac mechanism that causes this type of cardiac irregularity—incoordinate contraction or fibrillation of the auricular musculature—has been understood only during the past few years, its clinical prototype, the pulsus irregularis perpetuus of Hering, or the totally irregular pulse, has for years been recognized as one of the commonest as well as one of the gravest forms of cardiac arrhythmia. Perhaps from the widespread use of Hering's terminology, however, the erroneous impression has been prevalent that this condition once present is practically always permanent. Already a sufficient number of cases have been published to combat this view, so that we must now consider that auricular fibrillation, like the other disturbances of the