The pulmonary vein contains a myocardial layer connected to the left atrial myocardium, which is currently receiving attention as the source of ectopic electrical activity underlying atrial fibrillation. Isolated pulmonary vein preparations show automaticity, which is the result of spontaneous electrical firing of the myocardial layer. The firing is dependent on depolarizing membrane currents enhanced by elevated intracellular Ca²⁺ concentration. The weak repolarizing power of the pulmonary vein myocardium plays a permissive role in the manifestation of automaticity. The electrophysiological properties of the isolated pulmonary vein reflect the pathological status of the animal and are sensitive to various pharmacological agents. Thus, the isolated pulmonary vein is useful for the studies on the mechanism of atrial fibrillation and the evaluation of drug effects.