In pacing-induced models of atrial fibrillation (AF) that mimic atrial high-frequency foci, the increase in AF susceptibility over time is not paralleled by any increase in dispersion of repolarization (DOR). Measurements of effective refractory periods (ERP), however, were performed by using a single extra stimulus, which bears significant limitation because it cannot evaluate any dynamic increase in DOR that may arise at rapid heart rates. Repolarization alternans (Re-ALT), a beat-to-beat alternation in action potential duration, enhances DOR above a critical heart rate. It is unknown, however, whether Re-ALT plays a role in promoting AF. Method and results: two DDD pacemakers (Vitatron TM), each with right atrial (RA) and ventricular leads, were implanted in three male sheep (50-80 kg). The 1st pacemaker was used to deliver 1) electrophysiology protocols for measurements of RA ERP (S1S2) and of RA Re-ALT threshold (S1S1), and 2) intermittent RA burst pacing (for 5 sec followed by 2 sec of sinus rhythm) until sustained AF developed. The 2nd pacemaker was used to record a single broadband unipolar RA electrogram (EGM). RA repolarization wave (Ta) was indisputably seen following depolarization. In the first two sheep, sustained AF was induced after 4 and 6 weeks of RA burst pacing. RA ERP decreased progressively from 150 ms (pre-activation) to 125 ms and to 90 ms after 2 and 4 weeks of burst pacing respectively. Importantly, alternans of Ta was observed at fast pacing rates, but its threshold could not be determined because of 2/1 AV conduction with far field ventricular contamination of the RA Ta. The AV junction of the 3rd sheep was then ablated by RF in order to dissociate far-field ventricular activity from RA EGM. Re-ALT threshold was measured at 270 ms pre-activation, and remained stable until sustained AF developed. Importantly, the range of pacing CL during which RA Re-ALT was observed increased from 20 ms pre-activation to 140 ms before sustained AF developed. Of note RA Re-ALT appeared intermittently, but periods of Re-ALT increased in duration and amplitude as pacing CL decreased. Conclusion: we report here for the first time in vivo measurements of atrial repolarization alternans using standard pacemaker technology in a chronic sheep model of pacing-induced AF. Our findings suggest that atrial repolarization alternans might be a mechanism by which dispersion of repolarization transiently increases, promoting wavebreaks and AF at rapid rates.