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Department of Physiology, University of the Witwatersrand Medical School, Johannesburg, South Africa.

1. We measured rectal and hypothalamic temperature in sheep breathing nasally and via a tracheostomy, during hyperthermia resulting from exposure to a hot environment, exercise and fever. 2. In normothermic and hyperthermic sheep hypothalamic temperature was up to 1.0 degree C lower than rectal temperature when the sheep breathed nasally. Tracheostomy breathing abolished the rectal-hypothalamic temperature difference. 3. In sheep breathing via the tracheostomy and exposed to a dry-bulb temperature of 45-50 degrees C for 2 h, hypothalamic temperature exceeded rectal temperature by about 0.4 degrees C, and was significantly higher than that in sheep breathing nasally in the same environment. 4. During exercise on a treadmill and in the post-exercise period, the difference between hypothalamic and rectal temperature was abolished in the sheep while breathing through the tracheostomy, and rectal temperature rose to higher levels compared to those evident in the same activity while breathing nasally. 5. After an I.V. injection of 0.4 micrograms/kg lipopolysaccharide (LPS), the difference between hypothalamic and rectal temperature again was abolished in the sheep when breathing through the tracheostomy, but rectal temperature rose significantly less compared to when breathing nasally. 6. Our results indicate that selective brain cooling depends on upper respiratory tract cooling in normo- and hyperthermic states in sheep.

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