The maturation of the barrel field in the primary somatosensory cortex was observed in Nissl-stained preparations from rats ranging in age from 12 days to 1.5 years postpartum. Prior to the 20th day, the barrels in the rat resemble those of the mouse and have distinct cell-sparse hollows that are surrounded by cell-dense sides. They span the full thickness of layer IV. Between the 20th and 34th days, the barrels in only the posteromedial part of the barrel field gradually change and the distinction between the hollows and sides is lost throughout all but the deepest part of layer IV. The resulting mature barrels are relatively indistinct and have a uniformly high cell density that extends well into the supragranular layers. In contrast, the barrels in the anterolateral part of the barrel field remain essentially unchanged. The remodeling apparently is not due passively to cortical growth because, by P20, the thicknesses of the cortical layers and the dimensions of the barrels are virtually the same as in the adult. Several mechanisms are considered that may account for the changes. These include a redistribution of the neurons that originally were in barrel sides; a reduction in the neuropil between the neurons that originally were within hollows; and differential growth of layer IV dendrites. The changes in the barrel structure may be related to the differentiation and quantity of innervation in the hairy skin between the vibrissae.