Atrial fibrillation (AF) is the most common heart rhythm disorder. The presence of AF increases the risk of death and is associated with a 5-6-fold increase in stroke incidence, due almost exclusively to thrombus formation in the heart. Current therapies for AF are limited. The evaluation of new, more effective treatments for preventing AF recurrence remains a critical unmet clinical need. AF is considered a progressive disease that increases in prevalence with age and can convert from "paroxysmal" to "persistent" to "permanent" AF in a single individual. This progression results, in part, from high oxidative stress and progressive adverse electrical changes in the heart. Compelling preclinical and clinical data indicate that Relaxin, a naturally occurring peptide hormone, may reverse the electrical remodeling. Thus, our overall objective is to investigate the effects of Relaxin in Veterans who have failed medical management for symptomatic AF and is referred to Cardiac Electrophysiology Laboratory for catheter ablation and pulmonary vein isolation. We will determine whether Relaxin therapy, in addition to the standard of care, counteracts the oxidative stress-related electrical derangement and reduces the post-ablation AF burden. A unique aspect of this proposal is that it is based in part on observations derived from the basic, translational and computational labs of the PI and co-investigators and from the observations by the PI while caring for patients with AF. As such, this proposal represents a true progression from the bench to the bedside. If successful, our findings may lead to the design of a new, more effective treatment for a major unmet public health problem in the United States as well as the world.