Does the Invos near infrared spectroscopy monitor detect intracranial oxygenation, or reflect oxygenation of extracerebral tissues? The change in Invos regional cerebral oxygenation was studied during a doubling of middle cerebral artery flow with hypercapnia. There was no significant change in regional cerebral oxygenation, which suggests that the Invos does not reflect intracerebral oxygenation, possibly because of its narrow optode separation. The use of near infrared spectroscopy in adults may require a more powerful machine.