ABSTRACT Aging is associated with increased risk of developing atrial fibrillation (AF). While AF affects <0.2% of people under 49 years old, it occurs in ~6% of those over age 65 and ~9-17% in those over age 80. AF is associated with increased morbidity and mortality through stroke, myocardial infarction, and heart failure and is characterized by abnormal and rapid atrial electrical activity that arises due to adverse electrophysiological, Ca2+- handling, structural, and autonomic remodeling. Despite age being a risk factor for the development of AF, how natural age-associated remodeling makes the atria more susceptible to arrhythmias is not fully understood. An additional layer of complexity is derived from potential sex differences in etiology. While males develop AF at an earlier age, women with AF have worse symptoms, poor treatment outcomes and higher mortality. These differences in the disease state may be reflective of sex differences in baseline atrial electrophysiology and the age-associated remodeling that predisposes to AF. However, due to historic underrepresentation of females in scientific and preclinical studies, sex differences in atrial electrophysiology associated with natural aging are currently unclear. In line with the goals of the National Institute on Aging (NIA) to better understand the biology of aging and disparities in age-related disease, the aim of this R03 Small Research Grant is to investigate sex differences in atrial electrophysiological, structural, and autonomic remodeling in aging to determine how biological differences in healthy aging may be predictive of arrhythmia risk. To do this, we will use a translationally-relevant ‘middle-aged’ rabbit model that demonstrates age-associated changes that predispose to AF, but is a timepoint before late-stage remodeling. We will take an integrative approach performing functional in vivo, ex vivo and biochemical studies on 3-4 year old male and female rabbit hearts alongside complementary computational modeling. Aim 1 will examine sex-specific electrophysiological characteristics and arrhythmia susceptibility in the middle-aged atria through optical mapping, in vivo ECG recording, and quantification of protein expression and fibrosis. Aim 2 will examine changes in autonomic modulation in middle-age and its role in atrial arrhythmogenesis. Aim 3 will investigate ionic parameters that may predict arrhythmia risk. Together, these experiments will characterize electrophysiological and autonomic changes in aging, evaluate age- dependent arrhythmogenic mechanisms, and uncover novel predictive features underlying the transition from healthy aging to AF.