This paper describes the results of clinical and experimental studies which attempt to clarify the nature of the vascular changes in migraine. The preheadache phenomena of migraine are probably due to cranial vasoconstriction. These phenomena can be abolished or modified by the administration of a mixture of ten percent carbon dioxide with ninety percent oxygen during the preheadache or early headache phase. During the headache phase there is vasodilatation and edema of the vessel walls. When vasodilatation and edema were induced in patients experimentally they did not produce pain. It is therefore inferred that in addition other factors are necessary for the production of vascular headache. The vasodilatation may be an attempt on the part of the body to restore the homeostasis of cranial circulation.