&NA;With a closed head primate stroke model, acute cerebral ischemia limited to the middle cerebral artery (MCA) territory was produced by macrosphere embolization of the internal carotid artery bifurcation. Measurements of the oxygen tension (PO2) at the cerebral cortical surface were obtained by continuous on‐line mass spectrometry. Percentage of dry weight and tissue sodium, potassium, and chloride concentrations from ischemic and nonischemic hemispheres were determined at various times. With this preparation, we registered the precise onset of cortical surface PO2depletion, which showed an exponential downward trend (fast component from 0 to 5 minutes, t½= 0.8 minute, rate of change = 89% per minute; slow component from 5 to 240 minutes, t½= 285 minutes, rate of change = 0.3% per minute). After the onset of cerebral ischemia, there was an immediate fall of the cortical surface PO2with reductions of more than 45% at 5 minutes before definite hemiparesis and electroencephalographic abnormalities were recognized. During the secondary phase from 5 to 240 minutes, the cortical surface PO2fell by only an additional 23% of the steady state. Even so, when cortical surface PO2was maintained at this critically low level, the earliest cerebral cortical edema was evident 180 minutes after MCA occlusion. Thereafter, progressive accumulation of edema fluid in the cortex (90 to 170.8 microliters per g of tissue) and in the white matter (19 to 46.2 microliters per g of tissue) was detected by the end of 240 minutes of cerebral ischemia.