Ten chloralose-anesthetized rhesus monkeys were exposed to a total of 10 krads of 6-MeV x-rays administered in 1000-rad increments at an average of 158 rads/min. A second group of three monkeys was exposed to 2.5 krads of pulsed gamma-neutron radiation. Statistical comparisons were made between each exposure group and a sham-irradiated control group consisting of seven monkeys. Tachycardia and hypotension developed after 1.1 krads of x-ray. With 2.5 krads of gamma-neutron exposure, tachycardia developed after 1 minute and hypotension developed after 2 minutes. Therefore, the effect of intermediate-dose-rate high-energy x-ray and pulsed gamma-neutron irradiation appear dependent only on cumulative dose. It was shown that damage to baroceptor reflexes may contribute to the development of hypotension. Exposure to 2.1 krads of x-ray resulted in a significant decrease of brain blood flow. In contrast to x-ray exposure, for the first 5 minutes after gamma-neutron irradiation, brain blood flow increased, but at 20-25 minutes postexposure blood flow had decreased significantly. Lack of compensatory changes in brain vascular resistance was attributed to direct damage to brain autregulatory mechanisms, and/or to the opposing effects of radiation-induced hypocapnia.

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