In the acidic lumen of pre-synaptic vesicles, the fluorescence of spH is only about 5% of its intensity at neutral pH. When synaptic vesicles with spH anchored to their lumenal face fuse with the plasma membrane to release their contents, the spH is exposed to the pH-neutral extracellular environmment and rapidly becomes highly fluorescent. All olfactory bulb glomeruli fluoresce, with varying intensity. The superficial nerve layer overlying the glomerular layer and comprising axons of olfactory sensory neurons is more weakly fluorescent. In vivo fluorescent imaging of the olfactory bulb demonstrates that odorant-evoked responses cause strong fluorescence of odorant-specific, bilaterally symmetric groups of glomeruli.