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1. Arterial plasma catecholamine concentrations, rectal temperatures and respiratory rates were measured in the unanaesthetized ox in a cool environment (15/11·5° C dry bulb/wet bulb (D.B./W.B.) in a hot, dry environment (40/22° C), in a hot, humid environment (40/39·1° C) and during localized heating of the hypothalamus.

2. There was no significant difference in the plasma catecholamine levels of animals exposed to a cool environment (15/11·5° C) and to a hot, dry (40/22° C) environment.

3. When the animals became hyperthermic in the hot, humid environment, considerable increases in the plasma levels of adrenaline and smaller increases in noradrenaline concentration occurred.

4. Bilateral adreno-medullary denervation abolished the changes in plasma catecholamine levels associated with hyperthermia.

5. Localized heating of the anterior hypothalamus in either a cool or a warm environment did not result in any change in plasma catecholamine levels.

6. It was concluded that the source of the increased levels of plasma catecholamines during hyperthermia was the adrenal medulla and that the stimulus for their release was a nervous one rather than direct chemical or thermal stimulation of the adrenal medulla. The cause of the increased sympatho-adrenal activity was not apparent, but neither thermal stimulation of cutaneous and hypothalamic receptors nor the associated changes in respiratory activity seemed to be implicated in the response.