A simple bedside technique utilizing a suction electrode allows the recording of monophasic action potential (MAP) from the intact human heart. An analysis of the right atrium MAP in patients with various arrhythmias suggests that its duration provides a useful index of myocardial refractoriness. Short MAP after conversion of arrhythmias may indicate the instability of the sinus rhythm. In the same way, it is possible to predict the maintenance of the sinus rhythm after DC shock. MAP recording provides an interesting method for analysis of the electrical events during atrial arrhythmias: atrial flutter and fibrillation, supra-ventricular tachycardias and premature beats.