Endurance exercise is associated with an increased risk of atrial fibrillation (AF). We previously established that adverse atrial remodeling and AF susceptibility induced by intense exercise in mice requires the mechanosensitive and pro-inflammatory cytokine tumor necrosis factor (TNF). The cellular and mechanistic basis for these TNF-mediated effects is unknown.We studied the impact of TNF excision, in either atrial cardiomyocytes or endothelial cells (using Cre-recombinase expression controlled by Nppa or Tie2 promoters, respectively), on the cardiac responses to six weeks of intense swim exercise training. TNF ablation, in either cell type, had no impact on the changes in heart rate, autonomic tone or left ventricular structure and function induced by exercise training. Tnf excision in atrial cardiomyocytes did, however, prevent atrial hypertrophy, fibrosis, and macrophage infiltration as well as conduction slowing and increased AF susceptibility arising from exercise training. By contrast, endothelial-specific excision only reduced the training-induced atrial hypertrophy. Consistent with these cell-specific effects of Tnf excision, the loss of TNF in cardiomyocytes was able to prevent the activation of p38MAPKinase, a strain-dependent downstream mediator of TNF signaling, without affecting the atrial stretch as assessed by atrial pressures induced by exercise. Despite TNF's established role in innate immune responses and inflammation, neither acute nor chronic exercise training caused measurable NLRP3 inflammasome activation.Our findings demonstrate that adverse atrial remodeling and AF vulnerability induced by intense exercise requires TNF in atrial cardiomyocytes whereas the impact of endothelial-derived TNF is limited to hypertrophy modulation. The implications of the cell autonomous effects of TNF and crosstalk between cells in the atria are discussed.Endurance sport is associated with atrial fibrillation (AF) and mouse models show intense exercise training promotes atrial hypertrophy, fibrosis, inflammation, and AF vulnerability, which requires the mechanosensitive inflammatory cytokine tumor necrosis factor (TNF). We demonstrate that Tnf ablation in atrial cardiomyocytes protects fully against atrial changes induced by exercise, whereas endothelial-specific ablation only prevents atrial hypertrophy. Since atrial filling pressures increase markedly during exercise and most clinical conditions linked to AF (hypertension, heart failure, valvular/metabolic diseases), we discuss how atrial stretch may mediate cell autonomous effects of TNF and arrhythmogenic tissue changes in the atria.© The Author(s) 2023. Published by Oxford University Press on behalf of the European Society of Cardiology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.