The pattern of normal nerve fibers associated with barrel-shaped structures in the somatosensory cortex of both young and adult mice, has been studied using a reduced silver method. In adult animals, the [barrel] sides and septa can be seen to contain densely packed bundles of nerve fibers running vertically between layers III and V. In parasagittal sections, these fibers appear as very dark bands between adjacent barrels, while in tangential sections the fibers, cut in crosssection, appear as rings of dark spots concentrated around the barrel edges. In contrast to this, barrels in immature animals have dark, evenly stained centers and pale, cell dense sides. This immature pattern can first be distinguished in 2-day-old animals and persists until 18 to 19 days. Between 18 and 24 days a change from the immature to the adult pattern occurs with the appearance of darkly stained, fine fibers within layer IV, particularly within the barrel sides. It is suggested that the immature pattern is due, primarily, to the staining of thalamic afferents while the adult pattern appears with the development of intracortical and association fibers. Electron microscopy, on tissue previously treated by the silver method, shows that the silver deposits are mainly attached to longitudinal elements of the axoplasm and not associated with myelin. This may explain the success of this method in showing fibers in young, unmyelinated brains.