Hypocapnic-hyperventilation has a profound, but probably temporary, effect on CBF, producing approximately a 2% decline in CBF for each 1 torr decline in P co co2 . This effect appears to be mediated through changes in perivascular pH of the cerebral resistance vessels acting directly on the vessel wall. At low P co co2 the vasoconstrictor effect of short-term hypocapnic-hyperventilation is attenuated by resultant cerebral hypoxia. During prolonged hyperventilation CBF returns toward normal as the pH in the CSF is restored. Short-term hypocapnic-hyperventilation can be lifesaving in the treatment of acute intracranial hypertension. On the other hand, prolonged hyperventilation has not been convincingly shown to benefit patients, whether with severe head injury or cerebral infarction, or during carotid endarterectomy without bypass.