Total cerebral blood flow and blood flow in 10 brain structures of unanesthetized rats were measured by the indicator fractionation technique under different oxygen pressures. The first electrical discharge seen under high oxygen pressure was considered as an early sign of brain oxygen toxicity. It was found that the effect of high oxygen pressure on cerebral blood flow was time- and pressure-dependent. Exposure to 2 and 3.5 ATA of O2 led to vasoconstriction. Exposure to 5 ATA of O2 produced an initial vasoconstriction followed by a secondary vasodilatation to normal level. Cerebral blood flow was not reduced during exposure to 7 ATA of oxygen. Non-decreased blood flow preceded the appearance of the first electrical discharge in all investigated structures of the brain. There was no change in electrical activity of the brain as long as a decrease in cerebral blood flow was maintained. It is suggested that changes in electrical activity of the brain may be produced by toxic levels of brain tissue PO2. Vasoconstriction resulting in decreased cerebral blood flow was considered as a protective mechanism against the toxic effects of high tissue PO2.