We measured the input from the nose to the olfactory bulb in the mouse using calcium sensitive dye in the olfactory receptor cell nerve terminals in the glomeruli. We are now trying to measure the output of the bulb, carried by the mitral/tufted neurons, with the same glomerular level spatial resolution. The comparison of input and output would define the function(s) carried out by the bulb. Understanding the roles of different neuron types requires fluorescent protein activity indicators that can be expressed in specific cell types. The genetically encoded voltage indicator (GEVI) ArcLight consists of the voltage sensing domain of the Ciona voltage-sensitive phosphatase and the fluorescent protein super ecliptic pHluorin (A227D). The fluorescence of ArcLight changes by ~40% in response to a 100mV depolarization; about five times larger than previously reported signals. ArcLight can be specifically expressed in mitral/tufted neurons and reports odor-evoked electrical activity. The experiments comparing the input and output of the bulb are just beginning.