Nitroglycerin (NG) induces in cluster headache patients and controls an increase in systemic diastolic blood pressure and/or heart rate and a decrease in blood flow velocity in the middle cerebral artery (VMCA). Termination of NG induced cluster headache-like attack was correlated to an increase of VMCA compared to the VMCA before NG administration (p less than 0.01). This increase was not found in patients without attack or in controls. The NG induced "dynamite headache" in the controls subsided when blood pressure and heart rate were normalized, but the decrease of VMCA still prevailed. Orbital phlebograms have shown pathologic changes in cluster headache and in Tolosa-Hunt syndrome but not in controls. Ocular sympathetic nerves are involved in cluster headache but seldom in Tolosa-Hunt syndrome. It is suggested that the start of a cluster headache attack is due to an increase and the termination of the attack to a decrease of blood flow to the sympathoplegic phlebopathic cavernous sinus.