In past decades, many of the ablest medical investigators have attempted to resolve the mechanism of auricular fibrillation, the most important disturbance of the human heartbeat. The inconclusive nature of the results obtained by these workers undoubtedly is attributable to the fact that they did not utilize direct methods and their tools were not sufficiently sensitive to analyze the complex activity in the fibrillating auricles. For example, it it understandable that Sir James MacKenzie could not resolve the complex mechanism of auricular fibrillation from observations based solely on crude pulse tracings. The various earlier hypotheses were largely abandoned in 1920 when Sir Thomas Lewis, after extensive experimentation, advanced the circus movement theory. In the present investigation, the fibrillating auricle in man and animals has been reexamined with techniques not previously employed and apparatus not hitherto available. By these new methods the circus movement theory of auricular fibrillation has been found