Atrial and ventricular ectopies are common in dogs diagnosed with heart failure secondary to dilated cardiomyopathy (DCM). They cause irregularities of the cardiac rhythm that further deteriorate cardiac systolic function, and over time they can progress to sustained tachyarrhythmias, such as atrial fibrillation and ventricular tachycardia. Atrial fibrillation and ventricular tachycardia can lead to refractory heart failure, syncope and sudden cardiac death. Factors contributing to arrhythmias in the presence of heart failure include chamber enlargement, fibrosis, elevated adrenergic tone, renin-angiotensin-aldosterone system (RAAS) activation and electrolyte imbalances secondary to loop diuretic therapy. In people with heart failure and reduced systolic function, mineralocorticoid receptor antagonists decrease all-cause mortality, cardiac mortality and sudden death by 15 to 30%, and they have a protective effect against arrhythmias by directly targeting their substrate and triggers (Zannad F, 2012). The antiarrhythmic properties of spironolactone have not been studied in dogs with heart failure and a naturally occurring DCM phenotype. The goal of this pilot study is to establish initial estimates of the effects of spironolactone on arrhythmia burden, markers of arrhythmogenic risk and heart rate variability to inform the design of a larger trial. These preliminary results could also indicate a role for spironolactone as an add-on to standard antiarrhythmic drugs.