The size and distribution of mitral cells in the olfactory bulbs of rats were determined using a computer-assisted morphometric technique. Rats were reared from 2 weeks of age for 10 weeks in an animal colony, or in cages through which a stream of deodorized air or cyclohexanone vapor passed. Statistical analyses indicated that each treatment produced a different distribution of cell sizes in the coronal but not along the rostrocaudal plane of the bulb. The mean size of mitral cells in rats reared in deodorized air was substantially smaller than that of normal rats, and the mean length of their mitral cell layer in the coronal plane was less than in the other groups. Overall, these findings indicate that mitral cells which were of normal size in rats exposed to cyclohexanone had been stimulated by this odor. Cells which had not been stimulated by an odor in either of the experimental groups were smaller than normal and were degenerating or underdeveloped. The results also suggest that there are bands of mitral cells aligned along the rostrocaudal axis of the olfactory bulb which are responsive to specific odors and other evidence is presented which supports this view. The functional significance of exposure effects and their implications for the spatial coding of odor quality are discussed.