Carbocyanine dyes, particularly thiacyanines such as DiSC3(3) can inhibit respiration and may therefore be relatively cytotoxic. 3,3′-dipropyl-thiadicarbocyanine iodide is sensitive to membrane potential, fluorescence response to depolarization depends on the staining concentration and detection method. Selected applications are: -Calcium channels and other ion transport systems -Mitochondrial activity -Neurons and brain tissue -Membrane potential in intact yeast cells