Biomathematical models and experiments have indicated passive extracellular conductors influence field stimulation. Since metallic conductors prevent optical mapping under the conductor, we have evaluated a passive translucent indium tin oxide (ITO) thin film conductor to allow mapping of transmembrane potential (Vm) and stimulatory current under the conductor. A 1-cm ITO disc was patterned photolithographically and positioned between 0.3 cm² mesh shock electrodes on the ventricular epicardium of isolated perfused rabbit hearts stained with di-4-ANEPPS. For a shock (1 A, 10 ms) during the action potential plateau, optical maps from fluorescence collected using emission ratiometry (excitation at 488 nm and emissions at 510-570 nm and >590 nm) indicated the disc altered Vm by as much as the height of an action potential. The Vm became more positive near the edge of the disc where the ITO conductance gradient was parallel to applied current, and more negative near the opposite edge where the gradient was antiparallel to current. For diastolic shocks, the disc expedited membrane excitation at the sites of positive Vm in the heart and in a cardiac model with realistic ITO disc surface and interfacial conductances. Optical maps of ITO transmittance and the model indicated the disc introduced anodal and cathodal stimulatory current at opposite disc edges. Thus, ITO allows study of the stimulatory effects of a passive conductor in an electric field.