The isolated left atrium of the rabbit, which showed no spontaneous activity, was electrically driven for 20 beats with a cycle length of 500 msec. Tachycardia could be repeatedly initiated by the application of a single adequately timed stimulus shortly after the refractory period of the last basic beat. After the termination of the tachycardia, either spontaneously or artificially by a properly timed stimulus, this procedure was repeated. The number of beats of these tachycardias varied from just one (coupled extrasystole) to many hundreds. Surface electrograms were recorded at about 300 different sites. From the moments of activation of these sites, the spread of activation during regular driving and during the premature beat and the subsequent tachycardia could be determined. In contrast to the radial spread of the activation during basic rhythm, the impulse of the premature beat was propagated in a circular pathway. This circus movement was maintained during tachycardia. These results show that even in a small area of atrial muscle containing no anatomical obstacle the impulse can be entrapped in a circus movement. This circus movement was the underlying mechanism of the arrhythmia.