By using the newly developed voltage sensitive dye VF2.1.Cl invented by Miller and colleagues, I monitored simultaneously the spontaneous electrical activity of approximately 80 neurons in a leech ganglion, representing around 20% of the entire neuronal population. I was able to obtain long optical recordings for several minutes. I studied, also, the evoked response of nervous system by stimulating mechanosensory neurons. This work paves the way for further studies of multicellular networks using the new voltage sensitive dye.