1. Changes in afferent activity in the aortic and sinus nerves with alterations of blood pressure were studied in anaesthetized dogs. Mean aortic blood pressure was changed from 220 to 50 mmHg by using a pressurized reservoir connected to the abdominal aorta.
2. The stimulus-response curve (defined by measuring the mean impulse frequency at various pressures) from both nerves was S-shaped; the curve for the aortic nerve was shifted to the right of the sinus nerve curve.
3. In the dog, the aortic arch baroreceptors act predominantly as an anti-hypertensive mechanism; at lower pressures, the major control occurs through the sinus baroreflex.