As part of a larger study of the development of postnatal rat cerebral cortex, this report deals with the maturation of the cortex as a tissue from birth to twenty-one days of age. The changes in the numerical density of cells and blood vessels and the thickness of the cortex at successive ages were followed on light micrographs, and both were related to the ultrastructural observations on routine electron microscopic preparations. The maturation of the cortex is divided into two periods: The first ten days during which the growth to adult dimensions occurs and few patent blood vessels but large extracellular spaces are to be found, and the second ten days in which the majority of the vessels develop patent lumina, the perivascular sheath of astrocyte end-feet develops, and the large extracellular spaces disappear concomitant with the maturation of the neuropil.

The blood vessels appear to develop during the first ten days as solid cords of mesodermal elements which form a network of primordial vessels with thick walls and slit-like lumina containing a flocculent material. The formation of a blood filled, patent lumen apparently occurs synchronously over a brief period of time for the majority of the vessels during the early part of the second ten-day period. The basal lamina is ill-defined initially but takes on mature appearance parallel with the development of the perivascular glial sheath.

The volume of the extracellular space was found to be largest during the phase of rapid growth of axons and dendrites when few patent vessels exist. The disappearance of these large extracellular spaces during maturation of the blood vessels and neuropil is discussed in terms of the possible artifactitious nature of these spaces in both immature and mature cortex.