Sweating and flushing of the forehead and cheeks in response to body heating, embarrassment and strong gustatory stimulation were investigated in 23 patients with a unilateral lesion in the sympathetic pathway to the face. A lesion anywhere along this pathway impaired thermoregulatory sweating and flushing on the denervated side of the forehead in most patients and also of the cheek in some cases. Emotional sweating was also diminished on the denervated side of the forehead irrespective of the site of lesion, but impairment of emotional vasodilatation was noted only with peripheral (second or third neuron) lesions. These findings suggest that sympathetic vasodilator fibres accompany vasoconstrictor and sudomotor fibres through conventional sympathetic pathways to the face. Gustatory sweating and flushing were symmetrical in most patients but flushing was more marked on the denervated side in three cases. Gustatory sweating was accompanied by flushing on the denervated side of the forehead in one patient following section of the T2 and T3 roots. It is concluded that the cervical sympathetic outflow is the main pathway for thermoregulatory flushing and emotional blushing and that diminution or absence of such vasodilator reactions is a usual component of Horner's syndrome unless the responsible lesion is confined to the first thoracic root. Gustatory vasodilatation and sweating is preserved and becomes exaggerated in some instances.