The exact positions of microelectrodes used to measure the PO2 in the cerebral cortex of the rat were determined by staining the tissue with Alcian Blue. The measurement sites were subsequently located under a light microscope and correlated with the capillary and cellular arrangement of the cortex. The microelectrodes used for the PO2 measurements were made of gold glass fibers; the Alcian Blue was injected hydrostatically through a micropipette attached to the PO2 microelectrode. The sites where dye had been deposited were seen under a light microscope as green blue spots about 100 micrometers in diameter. The capillaries were visualized by silver nitrate perfusion. Differences between the local PO2 values in the neo- and the archeocortex were found. In the neocortex the mean PO2 was 31 mm Hg, capillary volume 1.6%, capillary surface area 980/mm2, capillary length 13.5/mm; whereas in the archeocortex these values where 21 mm Hg, 0.9%, 820/mm2 and 9.4/mm respectively. These data indicate a relationship between the microcirculatory transport system and the local oxygen tension and provide further evidence that the mean PO2 level tends to decrease when moving from the surface into the archeocortex.