In mammals, odorant molecules are thought to activate only a few glomeruli, leading to the hypothesis that odor representation in the olfactory bulb is sparse. However, the studies supporting this model used anesthetized animals or monomolecular odorants at limited concentration ranges. Using optical imaging and two-photon microscopy, we found that natural odorants at their native concentrations could elicit dense representations in the olfactory bulb. Both anesthesia and odorant concentration were found to modulate the representation density of natural odorants.