In the SmL cortex of mice and rats there are cytoarchitestonically identificable groups of cells -- called barrels -- some of which have been shown to be directly related to whiskers and other sensory hairs on the contralateral face. In this study we have used a comparative approach to determine the incidence and variation of the barrels. The brains of 27 mammalian species have been examined histologically to determine whether barrels exist in layer IV of what is known or likely to be the face area of SmI. Thick sections (50-100 mum) were taken tangential to the pia overlying SmI and stained with thionin. The patterns of facial whiskers were also mapped by dissection of the facial skin. Barrels were seen only in brains of species belonging to three of the seven mammalian orders examined. We have confirmed Weller's ('72) observation of barrels in the Australian brush-tailed possum but have not found barrels in two marsupials from the western hemisphere. Barrels were demonstrable in representatives of four of five rodent suborders examined and in the rabbit. From the study of the rodent brains, a number of trends emerge. (1) The organization of the barrel fields is "dictated" by the organization of the sensory periphery. Animals with five rows of large mystacial (moustache-like) vibrissae have five rows of PMBSF (Posteromedial barrel sub-field) barrels. (2) The barrels are confined to layer IV of (what is known or likely to be) the SmI face area. The pattern and cortical location of the barrel field is consistent among different specimens of the same species. (3) Certain behavioral patterns do not preclude the existence of the barrels. Species which possess well developed visual systems and behaviors (e.g., grey squirrel) and forms which do not actively explore the environment by whisking their vibrissae (e.g., guinea pig) have barrels. (4) Within a given rodent suborder, the barrels become more difficult to identify, as the brains become larger. We have not yet been able to demonstrate barrels in the largest rodent, the capybara.