1. The pattern of cardiovascular response evoked by carotid chemoreceptor stimulation has been investigated in cats anesthetized by continuous infusion of Althesin (Glaxo). 2. A variety of chemoreceptor stimulants, injected retrogradely into the lingual artery with the external carotid artery ligated, evoked hyperventilation with variable changes in arterial pressure and heart-rate, but a consistent vasodilatation in limb muscles and vasoconstriction in renal, mesenteric and cutaneous vasculature. 3. The muscle vasodilatation was still obtained after vagotomy and when the animal was paralysed and artificially ventilated; thus, it was not secondary to the hyperventilation. 4. In the majority of experiments the muscle vasodilatation was much reduced or abolished by atropine indicating it was mediated by sympathetic cholinergic fibres, which is characteristic of the alerting stage of the defence reaction in the cat. The cardiovascular pattern was accompanied by the other autonomic features of the alerting response, viz. pupillary dilatation, retraction of the nictitating membranes and pilo-erection. 5. In one and the same animal the pattern of response evoked by carotid chemoreceptor stimulation was the same as that evoked by noxious cutaneous stimulation, and by electrical stimulation in the brain stem defence areas. 6. It is concluded that peripheral chemoreceptor stimulation acts as an excitatory input to the hypothalamic and brain stem defence areas and that it can readily evoke the autonomic components of the alerting stage of the defence reaction. It is suggested that this has been missed in previous studies on anaesthetized animals because of the depressant action of chloralose and barbiturates on transmission in the hypothalamus and mid-brain.
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