Oxonol V is a sensitive slow-response membrane potential probe that is widely used for measuring membrane potentials of many biological systems. The fluorescence of Oxonol V decreases upon membrane hyperpolarization. In general, slow-response probes exhibit potential-dependent changes in their transmembrane distribution that are accompanied by a fluorescence change. The magnitude of their optical responses is much larger than that of fast-response probes (typically a 1% fluorescence change per mV). Slow-response probes, which include cationic carbocyanines, rhodamines and anionic oxonols, are suitable for detecting changes in average membrane potentials of nonexcitable cells caused by respiratory activity, ion-channel permeability, drug binding and other factors.