Mouse olfactory receptor proteins have relatively broad odorant tuning profiles, so single odorants typically activate a substantial subset of glomeruli in the main olfactory bulb, resulting in stereotyped odorant- and concentration-dependent glomerular input maps. One of the functions of the olfactory bulb may be to reduce the extent of this rather widespread activation before transmitting the information to higher olfactory centers. Two circuits have been studied in vitro that could perform center-surround inhibition in the olfactory bulb, one circuit acting between glomeruli, the other through the classical reciprocal synapses between the lateral dendrites of mitral cells and the dendrites of granule cells. One unanswered question from these in vitro measurements was how these circuits would affect the response to odorants in vivo. We made measurements of the odorant-evoked increase in calcium concentration in the olfactory receptor neuron terminals in the anesthetized mouse to evaluate the role of presynaptic inhibition in reshaping the input to the olfactory bulb. We compared the glomerular responses in 2- to 4-wk-old mice before and after suppressing presynaptic inhibition onto the receptor neuron terminals with the GABAB antagonist, CGP46381. We find that the input maps are modified by an apparent center-surround inhibition: strongly activated glomeruli appear to suppress the release from receptor neurons terminating in surrounding glomeruli. This form of lateral inhibition has the effect of increasing the contrast of the sensory input map.