Patterns of cytochrome oxidase (CO) activity were examined histochemically in the rat SmI cortex. Discrete regions of high enzymatic activity were centered upon the granule cell aggregates (barrels) in layer IV. Those barrels which correspond to the mystacial vibrissae and make up the posteromedial barrel subfield (PMBSF) were especially interesting in that CO staining revealed distinct metabolic subdivisions which do not have an easily demonstrable cytoarchitectonic counterpart. By analogy with the barrels in mouse PMBSF and with the cytoarchitectonically distinct barrels representing the smaller sinus hairs in the rat we propose that regions of high CO activity denote the "hollow" of the rat PMBSF barrels. In accord with previous physiological studies demonstrating a vertical organization in the rodent barrel cortex, we also noted columns of intense CO activity extending from layer VI through sublamina Vb. The centers of these columns coincided with the centers of the barrels in layer IV. In tangential sections through the infragranular laminae the segmentation of CO-positive zones was less distinct than in layer IV and appeared as bands of heightened activity oriented like the five rows of layer IV barrels. Highly reactive somata and dendrites were observed in both the granular and infragranular CO barrels indicating that some of the increased activity of these regions reflects oxidative metabolism of cortical neurons per se. These patterns of metabolic activity underscore the vertical and horizontal organization of the SmI vibrissa cortex and suggest that neurons located within the central core of a column have functional properties distinct from those located in zones where individual columns interface.