To test the influence of arterial O2 saturation (SaO2) on heart rate in cluster headache, changes in pulse rate induced by hyperoxia and hypoxia were monitored in 11 cluster headache patients (6 during cluster period, and 5 during remission). The results were compared with those obtained in 11 age and sex matched healthy individuals. The subjects were administered 5 min each of 100% O2 and 12% O2 in nitrogen in sequence. The aim of the latter procedure was to reduce SaO2 to approximately 80%. Pulse rate was recorded every minute from a finger pulse oximeter during the whole procedure. Cluster headache patients, in particular during the bout, had a slightly lower basal pulse rate than controls (P > 0.5, Student's t-test). This tendency was maintained throughout the test. Hyperoxia and hypoxia resulted in a marked, significant decrease and increase, respectively, in pulse rate from baseline values within each group. However, the difference between groups was not significant at any stage. Cluster headache patients therefore seem to have the same heart rate response to changes in SaO2 as healthy individuals. The marked heart rate changes which sometimes accompany cluster headache are unlikely to be caused by SaO2 changes.