We have determined, in vivo and simultaneously, the tissue PO2 (Ptio2) and the oxidation-reduction (redox) state of cytochrome aa3 (cyt aa3) of cat cerebral cortex during and after a short period of N2 breathing. Thirteen cats were anesthetized, ventilated mechanically with room air, subjected to a limited bilateral craniotomy, and then injected with 25 mg/kg of pyrenebutyric acid (PBA) intravenously. Ptio2 was measured from PBA generated fluorescence, emitted by monitored cerebral cortical cells. The cyt aa3 redox state was measured from differential absorption of monochromatic light at 605 vs. 590 nm reflected from the same cortical cells. In response to a 1.5-min N2 ventilation (phase I) the increase in PBA fluorescence signal, indicating a decline in Ptio2, lagged behind the cyt aa3 reduction. When the animal was ventilated with room air (phase II), rapid reoxidation, followed by hyperoxidation of cyt aa3 occurred. The decrease in PBA fluorescence signal, indicating an increase in Ptio2, was seen to lag behind cyt aa3 reoxidation. These results indicate that hysteresis exists in the relation between Ptio2 and cyt aa3 redox state. This may be the result of the situation that 1) low tissue O2 concentration is partially compensated by accumulation of reduced cyt aa3, and 2) following brief periods of anoxia, the affinity of cyt aa3 to O2 is increased.