Abstract: A catheter method for detecting intracardiac electrical potential fields makes use of a catheter provided with sensor electrodes which are connected through conductor wires to a signal receiver and processor. The catheter is inserted into the right or left ventricle through a hole formed in a vein or artery in a catheterization theatre, or during open thorax surgery, while normal cardiac function is maintained. The catheter has a distal end portion which may be inflatable, the size and shape of this end portion being such that the sensor electrodes are spaced substantially from the wall of the cardiac chamber in use of the catheter.