Summary A simple, safe and easily-reproduced method of recording the monophasic action potential (MAP) of the intact human heart is described. It involves the use of a special close bipolar electrode catheter. The advantages with this method compared with earlier described methods are pointed out on the basis of 57 investigations on 58 patients. Possibilities to analyze MAP from the right atrium and ventricle are illustrated in records of right atrial MAPs during sinus rhythm, atrial flutter, and fibrillation, as well as right ventricular MAPs during sinus rhythm. The method is promising in the explanation of electrocardiographical abnormalities.