Legend for cover figure. Optical mapping of rabbit epicardial transmembrane potentials produced by anodal and cathodal stimulation from a translucent indium tin oxide disc electrode. Location of disc edge is indicated by white dotted line. Indium tin oxide overcomes the problem of light blockage that occurs with usual metallic electrodes. Ratiometric measurements are obtained with transmembrane voltage-sensitive fluorescent dye and a laser scanner system that passes light through the electrode to interrogate underlying tissue. Stimulus-induced changes in transmembrane potentials are given as percentages of the action potential amplitude. Mean fiber direction of the tissue is indicated by arrow. Measurements reveal the transmembrane potentials are nonuniform with greater values in the direction parallel to the fibers.