In humans, electrical and contractile reverse remodelling following restoration of sinusrhythm (SR) after a prolonged period of spontaneous atrial fibrillation (AF), requires severalweeks. There is little known about this phenomenon in horses.In the present study, six healthy horses were instrumented with a neurostimulator and a pacemakerto maintain AF for four months by intermittent burst pacing and to study atrial andventricular electrophysiology. AF became persistent in all horses after two to six weeks of burstpacing. Before, during and after the AF period, parameters, such as the atrial fibrillation cyclelength, the right atrial and ventricular refractory period and vulnerability, such as inducingatrial arrythmias, atrial tachyarrythmias or maintaining AF, were determined. Two-dimensionalechocardiography was used to measure atrial and ventricular contractility expressed as fractionalshortening and size expressed as diameter and area.In two of the six horses, the procedure was discontinued due to an increased threshold (1horse) and due to infection at the level of the pacemaker pocket (1 horse). In the four remaininghorses, significant electrical and contractile remodelling compared to baseline values was observedfrom 48 hours onwards after AF induction. Upon restoration of SR with quindine sulfate,all electrical and contractile values returned to normal within one to two months. No ventricularremodelling was observed.Four months of pacing-induced AF resulted in electrical and contractile remodelling and reverseremodelling. The results suggest that pacing-induced chronic AF does not cause permanentdamage and suggest that a resting period of six to eight weeks before returning to training mightbe beneficial.