The hypothesis that instability of blood pressure occurring after carotid endarterectomy is related to interference with mechanical properties of the arterial wall in the region of the carotid sinus was investigated in 11 patients undergoing carotid surgery for transient ischaemic neurological disturbances. The operative procedure was found to change the mechanical properties of the arterial wall, there being an overall increase in diameter and reduction in distensibility, and these changes were reflected in the recorded change of spontaneous sinus nerve activity and the systemic blood pressure. It is suggested that the sinus nerve and carotid baroreceptor region should be carefully preserved during carotid endarterectomy to prevent reflex postoperative hypertension. Should hypotension occur in the postoperative phase it can be treated by local anaesthesia of the sinus region, anaesthetic blockade being maintained until secondary adaptation of the receptors takes place.