A method of locating the fossa ovalis in a patient by locating the His bundle, plane of the interatrial septum, and coronary sinus ostium in a patient, and thereafter locating the fossa ovalis on the basis of one or more predetermined distances between the fossa ovalis and the His bundle and the coronary sinus ostium. An apparatus for locating the fossa ovalis and performing a transseptal puncture is also provided.