Abstract: Compositions and methods for use in generating fast ratiometric voltage-sensitive fluorescence changes in single or multiple cells systems. A first reagent is a membrane-bound hydrophobic fluorescent anion which rapidly redistributes from one face of the plasma membrane to the other in response to the transmembrane potential, as described by the Nernst equation. A voltage-sensitive fluorescent readout is created by labeling the intracellular or extracellular surface of the cell with a second reagent comprising a fluorophore which can undergo energy transfer with the first reagent or a quencher for the first reagent. Quenching or FRET between the two reagents is disrupted when the membrane potential is depolarized, because the anionic first reagent is pulled to the intracellular surface of the plasma membrane far from the asymmetrically bound second reagent. In preferred embodiments of the invention, the first and second reagents are bound together by a suitable linker group.