It is known that application of anti-arrhythmic drugs for the acute treatment of arrhythmic can not only result in succesfull termination or prevention, but also can lead to unwanted pro- arrhythmic effects. On the basis of two arrhythmias, atrial fibrilation and Torsade de Pointes arrhythmias, we will highlight the relevance of differential atrial and ventricular electrical remodeling to explain the delicate and dynamic balance between anti- arrhythmic efficacy and pro- arrhythmogenic consequences of class III anti- arrhythmic drugs.