Photoplethysmography was used to record pulsations from the superficial temporal artery and supraorbital region in 23 patients suffering from cluster headaches, in 10 patients between bouts of headache, and in 10 control subjects. Pulsations were measured before and after the administration of 0.9 mg nitroglycerin sublingually, at the onset and height of cluster attacks and after inhalation of 100% oxygen for 10 minutes. Following nitroglycerin administration, increases in superficial temporal artery pulsations were greater on the symptomatic than on the nonsymptomatic side in patients who later developed headache. This persisted during cluster headache. By contrast, the increase in superficial temporal artery pulsations was greater on the nonsymptomatic side in patients who did not develop headache. The cause of this paradoxical response is unclear. Inhalation of oxygen produced significantly greater reduction of supraorbital pulsations on the symptomatic side in patients with headache. Decreases in both arterial territories following oxygen inhalation were significantly greater during cluster headache than between bouts or than in control subjects.