THE PHENOMENON of cardiac fibrillation has been known for more than a century. Its discovery immediately aroused considerable interest, but for several decades before 1946 little attention was paid to the mechanism of this arrhythmia. There were several reasons for this. The circus movement, or reentry mechanism, was accepted by many as a proved fact in flutter and fibrillation. In many books and articles it was stated categorically that flutter and fibrillation are due to a circus-movement, and objections against the theory were not even mentioned. Another obstacle to the study of their mechanism was the difficulty in obtaining flutter and fibrillation in the experimental animal in a practical manner. Ventricular fibrillation can be easily elicited by electrical stimulation. However, it is fatal within a few minutes in those larger mammals which are suitable for research. The cost was excessive in comparison to the briefness of the time available for