We hypothesized that infusion of VIP may induce headache in healthy subjects and that VIP induced headache may be associated with dilatation of intra- and extracerebral blood vessels. To test this hypothesis, we performed a double blind placebo controlled crossover study in normal human volunteers and studied the effect on headache and cerebral as well as hemodynamic parameters. In a double-blind, placebo-controlled, crossover design, the subjects were randomly allocated to receive 8 pmol/kg/min VIP or placebo (isotonic saline) over 25 min. Headache intensity, Mean velocity of blood flow in the middle cerebral artery (VmeanMCA), superfical temporal artery diameter, PetCO2, adverse events and vital signs were recorded at T-10, T0 and then every 10 min until 120 min after start of infusion. Single photon emission computerized tomography (SPECT) was performed at T-0, T20 and T60.