D ESPITE interest in the use of hyperbaric oxygenation in the management of certain disorders of the nervous system,S,ll,lz,15,17-~~ '~s the effect of high pressure oxygen on cerebral tissue oxygenation is not entirely clear. Some authors ~~ believe that relief of cerebral anoxia may be negated by the decrease in blood flow. Others 15'1s,24,34,'~s are of the opinion that total oxygen to the brain is increased, despite the presence of vasoconstriction. Previous experiments in animals with polarographic electrodes inserted into cerebral or spinal cord tissue '-',21,~9,~5 or on the surface of the brain 3 indicate a marked rise of pO~ with hyperbaric oxygenation. However, direct measurement of oxygen tension by needle electrodes inserted into nervous tissue has significant theoretical and technical difficulties, which limit its usefulness? ,9,~ Thus, attention has been directed toward cerebrospinal fluid pO~ as an indicator of cerebral tissue oxygenation. Bloor, et al., ~ and Jarnum, et al., '~'-' believe that the pO2 of cisternal cerebrospinal fluid is a reflection of average cerebral oxygen tension or available oxygen to the brain. Similarly, Gleichmann, et al., 16 found that direct polarographic measurement of pO~ in the subarachnoid space over the cerebral surface corresponds fairly well to the theoretical average brain tissue oxygen tension based on calculations for a tissue cylinder gas diffusion model.2~,25, ,~T Investigation of the effects of hyperbaric oxygenation on cerebrospinal fluid pO2 is therefore of interest. The present report attempts to correlate cerebrospinal fluid pO2 with changes in arterial oxygen tension in man. Only a few previous studies have dealt with the relationship of cerebrospinal fluid and arterial pO2 in patients a~176 and these have been carried out under normobaric conditions.