Golgi-Cox-impregnated neurons in the barrel cortex of the rat were studied qualitatively and quantitatively. Adult rat brains were sectioned perpendicular to or parallel to the cortical representation of the large facial vibrissae at 125 micron. Cortical laminar and barrel boundaries were identified from the Nissl counterstain. Over 200 well-impregnated neurons in cortical layers I-IV were selected for classification and further detailed study. Three broad classes of neurons were recognized: (1) pyramidal cells with conical somata, a stout apical dendrite, and spines; (2) class I nonpyramidal cells having small spherical somata and spiny dendrites; and (3) class II nonpyramidal cells having larger ellipsoid somata and smooth or beaded dendrites. The class I cells were further subdivided into "star pyramids" (cells with an apical dendrite) and spiny stellate cells (cells in which all dendrites were of similar length). The class II cells also were subdivided into multiform cells (with multiple dendrites radiating from the soma) and bipolar cells (with two principal dendritic trunks arising from the superficial and deep aspects of the soma). The position of these various cell types in the superficial cortical laminae was mapped in sections normal to the pia. Numerous examples of the class I and class II neurons were drawn with respect to the barrels in layer IV and the extent of their processes noted. Finally, approximately 250 barrel-related class I and II neurons were studied quantitatively using a computer-microscope and digitizing tablet. The density of the Golgi-impregnated neurons corresponds to the pattern of cell density seen with the Nissl counterstain. The various cell types are not uniformly distributed as a function of cortical depth. Cells with apical dendrites were found principally in the supragranular layers and star pyramids in the superficial one-half of layer IV. Spiny stellate cells are concentrated in layer IV and the smooth cells are present in greatest number in deep layer III and deeper layer IV. On the basis of these distributions we suggest that layer IV be subdivided into two sublaminae. The class I and class II neurons can be distinguished according to quantitative criteria which apply in either plane of section used. Class I neurons have smaller projected somal areas, more proximal dendritic branching, and shorter dendrites when class I and II neurons are measured in three dimensions.(ABSTRACT TRUNCATED AT 400 WORDS)