The abstract with its heading should not be more than 75 mm long. This is equivalent to 18 lines of text. Leave 1 line space at the bottom of the abstract before continuing with the next heading. The autonomic nervous system modulates atrial electrophysiology in atrial fibrillation (AF). The purpose of this study was (1) to non-invasively characterize the effects of exercise on atrial fibrillatory rate as marker of atrial refractoriness in patients with persistent AF and (2) to identify clinical and electrocardiographic predictors for rate response. In 15 patients with persistent AF, mean fibrillatory rate assessed by spatiotemporal QRST cancellation and time-frequency analysis remained unchanged with exercise. There were, however, 6 responders (rate change > 2.5%), with either a rate increase (N=5, 25±9 fpm) or decrease (N=1, -13 fpm). Absolute fibrillatory rate change (%) correlated inversely with baseline fibrillatory rate (r= -0.543, p=.045). In conclusion, sympathetic activation by exercise modulates atrial electrophysiology in some patients which can be monitored using time-frequency analysis. Higher baseline fibrillatory rates are associated with less autonomic modulation indicating advanced electrical remodeling.