Sixteen Macaca mulatta monkeys were exposed to a single 2500-R dose of cobalt-60 gamma irradiation at a dose rate of 500 R/min. Immediately following irradiation there was precipitous decline in cerebral blood flow. The initial decline was followed by a period of partial recovery within 20 to 30 minutes, with a subsequent slower secondary decrease in cerebral blood flow thereafter. Alterations in cerebral blood flow were accompanied by parallel changes in systemic blood pressure. Following irradiation there was a phase of immediate hypotension followed by transient partial recovery at 20 to 30 minutes. Contributing to the observed decrease in cerebral blood flow following irradiation was an apparent impairment of the normal regulatory function of the cerebral vasculature in maintaining adequate blood flow. The cerebral blood flow decline was of a degree that might be expected to produce impairment of normal CNS function. It occurred at a time during which behavioral incapacitation has been observed following supralethal doses of ionizing radiation, suggesting a causal relationship.

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