DiO/DPA membrane potential detection kit comprises a vial of the green fluorescent membrane dye DiOC16(3) dissolved in DMSO (1 mL at 2 mM) and a vial of the anionic membrane quencher dye DPA (dipicrylamine) dissolved in DMSO (1 mL at 20 mM). The DiO/DPA system detects cytoplasmic membrane potential changes using the principle of fluorescence resonance energy transfer (FRET), where DiO is a “stationary” FRET donor while DPA acts as a mobile FRET acceptor. The membrane location of DPA is a function of the polarity and magnitude of membrane potential, thus resulting in a membrane potential-dependent FRET efficiency. The DiO/DPA system has been reported to produce a fluorescence signal change of >56% in HEK-293 cells and >25% in neuronal cultures and brain slices per 100 mV membrane potential change (Bradley, et al. J. Neurosci. 29(29), 9197 (2009)). The DiOC16(3) in this kit is an improved version over the DiOC16(3) from other suppliers; our DiOC16(3) comprises gluconate as the counter ion, which makes the dye more water soluble, thus facilitating cell staining. DPA is traditionally a hazardous material due to its explosive nature. To improve its safety during shipping and handling, we have made it available as a diluted solution or as a solid in a smaller and more convenient packaging unit (See catalog no. 60037).Compatible with the NucleoCounter® NC-3000™.