Introduction: We recently observed in human left atrium periods of intermittent 2:1 and 1:1 atrial capture, preceded by atrial repolarization alternans (Re-ALT) during rapid pacing. It remains undetermined whether Re- ALT plays a role in preventing 1:1 atrial capture over long periods of rapid pacing. Methods: We specifically developed a chronic ovine model of rapid atrial pacing using two pacemakers (PM) each with a single right atrial (RA) lead separated by ~2 cm. The 1st PM was used to record a broadband unipolar RA EGM and the 2nd one to deliver rapid pacing protocols (400 beats) at incremental rates. Activation time (AT), activation recovery interval (ARI) and beat-to-beat differences in atrial T-wave apex amplitude (ΔTa) were analyzed until the 1st beat of 2:1 capture. Results: Intermittent 2:1 capture (panel A of figure) was observed in all sheep (n=9) at a mean pacing CL of 156±26 ms. 167 episodes of intermittent 2:1 capture were analyzed. Importantly, atrial Re-ALT (panel B of figure) was observed before 2:1 capture in 73% and AT prolongation in 55% of the sequences. Only 10% of sequences showed an absence of Re-ALT and AT prolongation. Conclusions: Using an ovine model of rapid atrial pacing, our findings suggest that Re-ALT may be a mechanism causing transition from rapid 1:1 atrial capture to 2:1 capture. Because rapid atrial tachycardia slows propagation velocity and promotes fibrillatory conduction, transitions to 2:1 capture may reduce susceptibility to atrial fibrillation.