Flexible cardiac electrode arrays are constructed utilizing integrated-circuit fabrication techniques. The use of a flexible polyimide substrate, Kapton (DuPont), offers two distinct advantages over conventional depth electrode arrays. Once the flexible arrays are implanted, they will not impede the normal contraction of the heart. Since the Kapton is an insulator, it interferes less with the fields in the heart which the array is designed to measure. Each multielectrode probe consists of an array of gold or silver electrodes supported on a 125-μm-thick Kapton substrate.