Intracarotid injection of 133-Xenon and recording of wash out of radioactivity by 254 external stationary detectors was used to measure rCBF. Initial slope values were calculated by a computer, translated into color code and displayed on a TV-screen. rCBF in patients with epilepsy has been the object of former studies. Those who had a cortical focus showed corresponding high blood flow values during attack and during EEG paroxysms without clinical attacks. On the contrary, all other patients with a cortical focus were normal. Patients with complex partial seizures displayed no abnormalities interictally. During generalized epileptic seizures rCBF and metabolism doubled. Distinction must be made between patients with aura (classic migraine) and patients without aura (common migraine). In the latter CBF was normal or slightly elevated. In the former the aura symptoms are associated with reduced CBF. The reduction usually starts posteriorly and spreads gradually anteriorly. rCBF remains depressed for up to several hours into the headache phase. After 8-12 hours a reactive hyperemia may ensue. In common migraine rCBF remains normal throughout the attack. During classic migraine attacks rCBF is depressed whereas it is increased during epileptic attacks. Thus rCBF studies further strengthen the differences between epilepsy and migraine.