Heart failure (HF) is a leading cause of mortality and is associated with cardiac remodeling. Vulnerability to atrial fibrillation (AF) has been shown to be greater in the early stages of HF, whereas ventricular tachycardia/fibrillation develop during late stages. Here, we explore changes in gene expression that underlie the differential development of fibrosis and structural alterations that predispose to atrial and ventricular arrhythmias. Overall design: RNA from canine right atria (RA) and right ventricle (RV) from controls and ventricular tachypaced to generate heart failure (HF) at early stages (2 weeks) and late stages (5 week) was isolated and gene expression was analysed by the Affymetrix (Canine_2 canFam2) chip (triplicates)