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1. The effect of moderate intensities of stimulation of the hypothalamic defence area on the baroreceptor reflex has been investigated in the cat by comparing the responses of arterial blood pressure and perfusion pressure of the isolated hind-limb muscle bed perfused at constant volume inflow, when the isolated carotid sinus was subjected to a series of non-pulsatile pressures with and without simultaneous hypothalamic stimulation.

2. In the absence of hypothalamic stimulation the characteristic sigmoid curves relating sinus pressure to blood pressure or muscle perfusion pressure were obtained.

3. With simultaneous stimulation of the hypothalamus a similar sigmoid relationship was found. There was no evidence of any reduction in the over-all power or maximum sensitivity of the baroreceptor reflex.

4. However, in those cats which had been atropinized to abolish the cholinergically mediated muscle vasodilatation, the curves obtained during hypothalamic stimulation were displaced in such a manner as to suggest that, while baroreceptor modulation of vasoconstrictor tone continued during defence area stimulation, the blood pressure regulating mechanism had been `reset' so that, within the physiological range of sinus pressures, any given level of sinus pressure was associated with a greater vasoconstrictor tone.

5. In non-atropinized cats there was little displacement of the curves relating sinus pressure to blood pressure, while the curves relating sinus pressure to muscle perfusion pressure were displaced in the opposite direction so that over-all muscle vascular resistance was less than normal at each level of sinus pressure.