Auricular fibrillation is a serious cardiac irregularity which results from the onset of one or more circus movements in the auricular musculature (Lewis). The factors that initiate the circus movements are not too well understood. In the following study these factors have been investigated, and it became clear that auricular fibrillation can be produced in hearts when they are subjected to the combined action of an exciting agent and overactivity of the vagus nerve. THE ACTION OF THE VAGUS Our first intimation of the nature of the physiologic mechanism involved in auricular fibrillation was obtained while observing the effects of acetyl-β-methylcholine chloride on a hyperthyroid patient in whom a tachycardia had suddenly developed. As a matter of fact, the patient was really suffering from auricular flutter, and administration of the drug converted the flutter into auricular fibrillation. The electrocardiogram obtained from this patient is reproduced in figure 1 and in