Natural atrial flutter was discovered In a dog. Two forms of the arrhythmia resembling the human counterparts of typical and atypical atrial flutter were noted. The Observations in this dog led to studies in a series of normal dogs in which the extrastimulus technique was used to evoke runs of repetitive activity simulating but unlike true atrial flutter. Epicardial atrial activation maps were made from 72 to 96 bipolar complexes recorded during the arrhythmias. Multiple effective refractory periods were determined and these values were used to construct maps of the refractory period distribution. Dog 1, with true atrial flutter, demonstrated a complex form of circus motion characterized by unidirectional block and one-way conduction. Abnormal slowing of the unblocked circus wave stabilized and maintained the repetition by permitting more time for recovery of a uniform state of excitability ahead of the wave. The slow conduction in this dog was associated with right atrial hypoplasia and discontinuity in the preferential atrial conduction pathways. The circus activation patterns in the dogs with evoked flutter were similar to those in true flutter; however, the cycle lengths were shorter and the circus wave conducted faster. In evoked flutter the regional differences in refractory period duration determined the one-way block and circus conduction patterns. The circus pattern was caused by a nonuniform bimodal refractory state of the atrium that simultaneously exerted a blocking effect while permitting conduction and complex shaping of the unblocked wave, which was routed back to its site of origin. Thus atrial flutter was the result of three Interacting factors: (1) an atrial premature beat, (2) nonuniform distribution of atrial refractory periods, and (3) slow conduction of the circus wave initiated by factors 1 and 2.