AbstractThe participation of aortic nerve C fibres in the baroreflex was investigated by recording changes in renal nerve activity in response to acute increases in arterial pressure, in two experimental situations: with the A and C fibres of the left aortic nerve intact, and with the A fibres temporarily blocked by a hyperpolarizing current (anodal block). Pressure was increased by manual inflation of an intra‐aortic balloon, and interference from other baroreceptor areas was avoided by carotid occlusion and sectioning of the right aortic nerve. Rises in mean arterial pressure exceeding 20 mmHg, to levels above 110 mmHg, were needed to trigger sympathetic inhibition via C fibres. A rise of 45 mmHg caused 50% reduction in sympathetic activity, equal to that obtained by stimulation of aortic nerve C fibres at about 3 Hz. In contrast, a rise of 20–30 mmHg evoked more than 60% reduction in sympathetic activity when the A fibres were operative. Judged by these studies of peak reflex responses to brief pressure rises, baroreceptors with C fibres in the aortic nerve have a much higher threshold to pressure than their myelinated counterparts; the C fibres contribute to the baroreflex inhibition of sympathetic discharge only when pressure is increased well above normal resting levels.Preliminary presentations of this paper were given in June 1976 at the 7th European Congress of Cardiology, Amsterdam, The Netherlands, and at the 8th Nordic Hypertension Meeting, Marstrand, Sweden (Aars, Myhre and Haswell 19