In this study the investigators will research the hypothesis that the gas carbon monoxide induces headache and increases the blood flow velocity in the middle cerebral artery.

To investigate headache score and accompanying symptoms during and after inhalation of carbon monoxide.

With transcranial Doppler imaging, c-scan and laser speckle the investigators will investigate changes in blood flow velocity of the middle cerebral artery (MCA), diameter of the superficial temporal artery (STA), the radial artery and facial skin perfusion.

Inclusion Criteria:

• Healthy volunteers
• Aged 18-50
• 50-100 kg

Exclusion Criteria:

• Tension type headache more than once/month
• Other primary headaches
• Daily medication except contraceptives
• Drug taken within 4 times the halflife for the specific drug except contraceptives
• Pregnant or lactating women
• Exposure to radiation within the last year
• Headache within the last 24 hours before start of trial
• Hypertension
• Hypotension
• Respiratory or cardiac disease
• Smoking
• Primary relatives with current or previous migraine