We have tested the feasibility of real-time three dimensional intracardiac echocardiography for guidance of interventional electrophysiology studies. We have constructed 2D array ultrasound transducers operating at 5MHz with 64 channels in a 12 F catheter, and of 137 channels in an 8 F catheter with a 12 F flexible interconnect. The system produces up to 60 3D scans per second and features real-time volume rendering. Using an open chest sheep model, we obtained real-time 3D images of anatomical landmarks, including the pulmonary veins and coronary sinus, which are of value in electrophysiological procedures.