An endocardial ablation apparatus, for introduction into a heart chamber formed by a wall, is provides. The ablation apparatus includes an inflatable, flexible porous membrane adapted to receive an electrolytic solution, and become inflated to substantially conform an exterior surface of the membrane to the wall of the heart chamber. An inner lumenal member is surrounded by and attached to the membrane. The inner lumenal member includes a lumen that permits blood flow through the inner lumenal member and heart chamber. An introducer catheter introduces the membrane and inner lumenal member into a selected heart chamber. A plurality of RF electrodes define a circuit positioned in the membrane or on an exterior surface of the inner lumenal member. The RF electrodes transfer thermal energy to the electrolytic solution. The electrolytic solution is the electrode that provides ablation of a selected site of the heart chamber. An RF power source is coupled to the RF electrodes. A source of electrolytic solution is coupled to the membrane.