The neurons in layer IV of the mouse somatosensory cortex are arranged in a remarkably consistent pattern of multicellular cytoarchitectonic units called "barrels." Each barrel is known to be related, in a one to one manner, to a contralateral whisker or vibrissa on the animal's face. In this study we have examined Golgi-impregnated neurons that comprise the "barrels." Several criteria, some being quantitative measures of dendritic arbors and somal sizes which were obtained with a computer-microscope system, suggest that all barrel neurons can be placed in two classes, the members of which are present in approximately equal numbers. The cells in the two classes can be further subdivided on the basis of the relationship of their processes to the barrels: 85% of them have processes restricted to a single barrel; 15% of them distribute their processes to two or more barrels. From these observations it is possible to suggest that a majority of neurons comprising the barrels would respond initially to movements of only one whisker while the remainder would respond to movements of two or more whiskers. In addition it has been shown that the quantitation of neuronal structure can provide a numerical basis for the classification of neurons in the central nervous system.