Rats were exposed to a moderate concentration of ethyl acetoacetate, amyl acetate, cyclohexanone or heptanol, for 69 days from postnatal day 1, whilst other rats were exposed to deodorized air, or the normal range of rat colony odors (Controls). The size of approximately 3% of all mitral cells in the olfactory bulb were measured using a computer-assisted method. As in previous studies in which stimulation began at day 14 or later, some regions of the olfactory bulb had cells that were significantly smaller than those of controls, and rats exposed to deodorized air generally had the smallest cells. Unlike in earlier studies, however, some regions of the olfactory bulb had cells that were significantly larger than similar regions of controls. The distribution of these changes appears to be odor-specific and consists of coronal patterns that extend throughout the rostro-caudal extent of the bulb. The different patterns obtained for the 4 odors used in this study provide evidence for a topographic coding of odor quality at the level of the mitral cells.