We measured vascular reactivity--expressed i) as decrease in blood flow velocity (cm/sec) per vol% CO2 decrease due to voluntary hyperventilation and ii) as increase of blood flow velocity during the first minute after resuming normal ventilation, per vol% CO2 increase--in the middle cerebral and basilar artery of 48 migraineurs with attacks without aura, and 17 normal controls. We found no differences for both determinants of vascular reactivity between migraineurs during and outside an attack, and between migraineurs and healthy volunteers. We conclude that the vasomotor reactivity is normal during migraine attacks without aura.