A four year old intact male Harlequin Great Dane presented for further evaluation of persistent atrial fibrillation. The major goal of the initial examination was to determine if the arrhythmia was a primary entity or present secondary to underlying cardiac disease. The results were equivocal and therapeutic planning was geared towards either a primary etiology or early secondary disease. Transthoracic electrocardioversion was performed and resulted in a temporary return to normal sinus rhythm (approximately four hours). Management efforts were then centered on pharmacologic rate control. The focus of this report will be on diagnosis, management, and etiology of canine atrial fibrillation. Emphasis will be placed on methods for restoration of normal sinus rhythm versus ventricular rate control.