Any effects of lithium carbonate (oral dosage 900 mg/day) on measurements of cerebral hemodynamics were examined in 10 carefully selected patients meeting criteria of chronic cluster headache, i.e., headache-free intervals were less than 3 weeks in duration per year. Cerebral blood flow (CBF) was measured by the 133Xe inhalation method before and 2-4 weeks after (mean 3 weeks) standard recommended lithium carbonate administration, as a dose of 300 mgs give orally three times per day (total 900 mg/24 hours).Cerebral vasoconstrictor responsiveness was tested by repeated CBF measurements during serial inhalations of 100% O2 followed by 5% CO2 in air mixtures. Mean values for hemispheric gray matter flow were 70.9 ml/100g brain/min measured over the side of the cluster headaches and 70.0 ml over the contralateral side prior to lithium and 68.0 ml for sides of the head after lithium treatment. These differences before and after treatment were not significant. Cerebral vasodilator responsiveness to 5% CO2 was expressed as per cent gray matter flow change per mmHg change of measured PECO2 . Compared to values measured in normal age-matched controls, CO2 responsiveness was diminished bilaterally, both prior to and during lithium administration and there were no significant changes induced after treatment with the drug. Likewise cerebral vasoconstrictor responses during 100% oxygen inhalation were not altered by lithium treatment.