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1. A method is described for isolation of the aortic arch and right subclavian—carotid angle in situ in the rabbit and perfusion with Krebs—Henseleit solution or blood under controlled conditions of pressure and temperature.

2. The characteristics of the baroreceptors of the aortic arch and right subclavian—carotid angle were studied by recording from single or few-fibre preparations of the left and right aortic nerves respectively. Curbes were plotted to show the relationship between the frequency of baroreceptor impulse activity and intra-aortic pressure during non-pulsatile perfusion under steady-state conditions.

3. The aortic arch and right subclavian—carotid angle baroreceptors were found to have similar characteristics. Three types of response of the baroreceptors at the threshold pressure to a steady intra-aortic pressure are described.

4. Increasing the intra-aortic pressure increased the frequency of impulses in fibres previously active and caused recruitment of other fibres in multi-fibre preparations. The relationship was linear at low pressures and a point of inflexion occurred at higher pressures in the majority of fibres.

5. Lowering the temperature of the perfusate reduced the impulse frequency at any given pressure.

6. The curves obtained during stepwise increases and decreases in intra-aortic arch pressure were dissimilar, particularly at the lower end of the pressure range. This phenomenon is probably due to properties of the arterial wall.

7. When the aortic arch preparation was excised, changes occurred in the shape of the impulse frequency—pressure curves from baroreceptors in both areas. The point of inflexion was elevated and a higher percentage of fibres failed to reach a point of inflexion in the pressure range studied.