These mutant mice express synapto-pHluorin (spH) from the endogenous locus encoding the olfactory marker protein (OMP) as a result of a targeted knockin mutation that replaces the OMP coding region with that of spH. The OMP locus directs expression of spH at high levels and exclusively in mature sensory neurons of the main olfactory epithelium (OSNs) and the vomeronasal epithelium. SpH is a pH-sensitive variant of the green fluorescent protein (ecliptic pHluorin) fused to the mouse synaptic vesicle-associated protein (VAMP2). The protein is localized preferentially in synaptic vesicles, but is also present on the plasma membrane of OSN axons and nerve terminals. The fluorescent domain of spH is exposed to the acidic lumen of synaptic vesicles where it is ~ 20 fold less fluorescent than at neutral pH. Upon synaptic release, the lumen of the synaptic vesicles becomes continuous with the extracellular space resulting in an increase in pH that causes a rapid increase in fluorescence. In mutant mice all glomeruli of the olfactory bulb are robustly labeled. There are ~1800 glomeruli in the main olfactory bulb, and each glomerulus receives axonal input from thousands of OSNs that express the same odorant receptor. Glomeruli in the dorsal surface of the olfactory bulb can be observed in vivo, after thinning the skull, through a standard epifluorescence microscope. Fluorescence changes caused by neuronal activity and presynaptic release can be measured in living mice following odorant stimulation. Mice that are homozygous for this allele do not express gene product (OMP) but are viable, fertile, normal in size and do not display gross physical or behavioral abnormalities. Homozygous mice produce small litters and breed at reduced frequency. This mutant mouse strain should prove useful to measure stimulus-induced activity or presynaptic vesicle release in OSNs of living mice.