Methods: 6 healthy volunteers will receive intravenous infusion of L-kynurenine using the following doses 12, 24, and 36 nmol kg-1 min-1 over 20 min on 3 different days with at least 1 day in between. Before and after infusion (at 20, 40, 60, 80 and 100 min) we will record vital signs, circumferences of middle cerebral artery, superficial temporalis artery and headache intensity and characteristics.