The fluorescence of an axon stained with a merocyanine dye was found to change during an action potential. The fluorescence change was directly related to membrane potential, and thus, in principle, fluorescence could be used in place of an electrode to measure membrane potential. We are hoping to develop an apparatus with many photodetectors that can monitor activity in many neurons simultaneously, using this fluorescent dye. We plan to use it on an invertebrate ganglion such as segmental ganglion of the leech, Hirudo medicinalis. We would like to construct a map of the functional connections between neurons and to use that map to study the neuronal basis of behavior.