Surgical manipulation in the region of the carotid sinus resulted in diffuse electroencephalographic slowing in two patients, despite the absence of bradycardia or hypotension. Direct observation confirmed the absence of carotid compression. The effect was abolished by infiltration of lidocaine hydrochloride into the carotid sinus area. Carotid clamping in one patient failed to produce similar electroencephalographic changes. We believe that this is the first observation of an influence of carotid sinus stimulation on human cerebral activity not attributable to reflex cardiovascular changes or complicated by the possibility of carotid compression.