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1. In the unanaesthetized rhesus monkey, 5-hydroxytryptamine (5-HT), catecholamines, acetylcholine or carbachol were micro-injected in a volume of 1·0 µl. or less through chronically implanted cannulae at eighty-six sites in the hypothalamus.

2. 5-HT in doses of 2-10 µg caused a long-lasting elevation in temperature which was dose-dependent. An anatomical `mapping' of the hypothalamus revealed that the hyperthermic action of 5-HT was localized to the anterior, pre-optic area directly ventral to the anterior commissure.

3. Noradrenaline in doses of 1-12 µg produced a dose-dependent fall in temperature of short duration. An anatomical `mapping' showed that the hypothermic action of this and other catecholamines was again localized to the anterior, pre-optic region.

4. Acetylcholine, alone or in a mixture with eserine, or carbachol caused a dose-dependent hyperthermia which was characterized by an intense rise of short duration and vigorous shivering. A `mapping' of this response revealed a diffuse patterning of sites throughout the hypothalamus which were sensitive to the application of acetylcholine and carbachol. However, in one circumscribed region at the junction between the posterior hypothalamus and mesencephalon, the two cholinomimetic substances caused a marked fall in temperature.

5. We conclude that 5-HT activates a cholinergic heat production pathway which projects from the anterior to posterior hypothalamus. Noradrenaline, on the other hand, blocks the hyperthermic action of 5-HT rather than activates the heat loss system. A chemically mediated heat loss pathway apparently does not exist in the hypothalamus.

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