Cerebral extracellular fluid (ECF) Po2, Pco2and pH were measured with cortical surface electrodes and HCO3-was calculated during acute hypoxia (Pao225–30 mm Hg). The values found did not correlate consistently with reduction of halothane requirement (MAC). In the first half hour of either nonnocapnic or hypocapnic hypoxia, MAC decreased to 76 per cent of control, unrelated to the ECF measurements. In the remaining hour and a half of normocapnic hypoxia (group 1), MAC decreased to 19 per cent of control while cerebral ECF pH fell to 6.93. Arterial oxygen content (Cao2) decreased as arterial pH fell. During the remaining hour and a half of hypocapnic hypoxia (group 2), MAC and Cao2remained constant while ECF pH decreased to 7.23. Hypoxia affected the Po2and HCO2-of ECF similarly in the two groups. Wide scatter of the data prevented meaningful correlation of MAC and cerebral ECF Po2, pH, and HCO2-in both groups. Cerebral ECF pH was not representative of cistemal CSF pH during hypoxia, as evidenced by a rapid and profound decrease in ECF pH while CSF pH remained fairly constant. The surface electrodes gave a closer measure of cerebral acid-base function than cistemal CSF. However, surface changes may reflect brain intra-cellular fluid acid-base function poorly, especially during periods of rapid change.