Two major causes of spoiled food smells such as fatty, fishy off-flavors are alkylamines liberated by bacterial actions and aliphatic acids-aldehydes generated by lipid oxidation. Using the method of intrinsic signal imaging, we mapped alkylamine-responsive glomeruli to a subregion of the aliphatic acid-responsive and aldehyde-responsive cluster in the odor maps of rat olfactory bulb. Extracellular single-unit recordings from mitral-tufted cells in the subregion showed that individual cells responded to the alkylamines in addition to acids and aldehydes. Responses of mitral-tufted cells tended to last for a long period (5-88 sec) even after the cessation of the alkylamine stimulation. These results suggest that the subregion is part of the representation of the fatty, fishy odor quality. Fennel and clove, spices known to add flavor and mask the fatty, fishy odor, activated glomeruli in the surrounding clusters and suppressed the alkylamine-induced and acid-aldehyde-induced responses of mitral cells, suggesting that the odor masking is mediated, in part, by lateral inhibitory connections in the odor maps of the olfactory bulb.