**Abstract**

Melatonin is a promising cardioprotective agent. Its increase during the night is associated with healthy cardiovascular function. On the other hand, reduced levels of melatonin are related to diseases. Aging and chronodisruptors reduce melatonin levels. Pharmacological supplementation reduces the deleterious effects of cardiovascular risk factors and improves the myocardial response to ischemia/ reperfusion injury and other proarrhythmic conditions. The protective mechanisms of melatonin involve its antioxidant properties as well as receptor-mediated actions. Signaling pathways include membrane responses, cytoplasmic modulation of kinases, nuclear receptor interactions, and improvement of mitochondrial functions. This chapter focuses on the electrophysiological and the antiarrhythmic properties of melatonin. The acute and chronic protective mechanisms of melatonin will be analyzed with an emphasis on transmembrane potentials and intercellular communication. An outstanding antifibrillatory effect makes melatonin a novel antiarrhythmic agent worthy of further exploration in the path to clinical applications.

**Keywords:** melatonin, arrhythmias, ventricular fibrillation, action potential, connexin 43, melatonin receptors
