Di-2-ANEPEQ is a fast-response, voltage-sensitive fluorescent dye used to monitor changes in membrane potential in living cells. It exhibits a shift in its excitation spectrum upon depolarization, enabling ratiometric imaging of electrical activity. This cell-permeant dye is particularly suitable for high-speed imaging applications in neuroscience, such as monitoring action potentials in neurons, and in cardiac research to study excitation-contraction coupling. It is also employed in studies of mitochondrial membrane potential and other cellular electrophysiology experiments with minimal phototoxicity.