An endocardial ablation and mapping apparatus is introduced into a heart chamber for mapping to detect arrhythmogenic foci, and ablate endocardium at the arrhythmogenic foci. An inflatable, flexible porous membrane is adapted to receive an electrolytic solution, and become inflated to substantially conform a conductive surface of the membrane to the wall of the heart chamber. A membrane support is surrounded by the membrane, and includes a sealed proximal end and a sealed distal end. Each end has an aperture formed therein defining a central lumen in the membrane support that permits blood flow through the support member and the heart chamber. The membrane support is attached to the membrane and is expanded to a non-distensible state when the membrane is inflated. A catheter, with a distal end, is attached to the membrane or the membrane support. The membrane and membrane support are introduced into the heart chamber by the catheter in a non-expanded state, and become expanded to an expanded state by inflating the membrane with the electrolytic solution. A plurality of treatment electrodes, defining a circuit, are formed on an exterior surface of the membrane support. An RF power source is coupled to the treatment electrodes, and a source of electrolytic solution is coupled to the membrane.