Abstract: An electrode balloon for use in anatomical cavities, such as cardiac chambers, to pick up bioelectrical signals from the walls thereof simultaneously in a multitude of sites for the mapping of the activation potentials thereof comprises an inflated tight rubber balloon having an opening at a proximal end thereof communicating with a filling tube which defines apertures for dispensing fluid in the rubber balloon. An extensible knitting which covers the rubber balloon is provided with a plurality of electrode heads distributed thereon to contact the walls of the cavity when the rubber balloon is inflated by a fluid conveyed therein through the tube. Insulated electrical conductors connect the electrodes to a signal recording and processing device.