A computer assisted method was used to measure the distribution of the size and number of cells in the mitral cell layer of ten olfactory bulbs from six Sprague-Dawley rats. The size of mitral cells was found to vary around the coronal but not along the rostrocaudal extent of the bulb. Cells were smaller in the ventral and dorsal aspects of the bulb, and larger in the medial and lateral aspects; possible causes of these differences are discussed. The pattern of cell size variation in the coronal plane was consistent both within and between animals, although the average size of mitral cells varied from rat to rat. These changes in mean cell size in the coronal plane were due to overall shifts in the size of cells and not to altered proportions of large and small cells. There was no consistent pattern however for cell density as measured by inter-cell distances of mitral cells, either in the coronal or the rostrocaudal planes. Estimates of the number of cells per bulb and the size of cells were observer dependent, whereas the pattern of cell size variation was observer independent. This characteristic pattern of size variation of mitral cells in rats reared in a normal rat colony environment may be due to a functional differentiation of mitral cells, as size has been shown to be associated with the amount of information processed by a cell. That mitral cells possess such a distinct pattern of size variation should therefore be considered if representative data are to be obtained from anatomical, electrophysiological and other studies of the olfactory bulb.