Cerebral oedema is an abnormal accumulation of fluid in the brain. In gray matter, this excess fluid appears in electron micrographs to be mainly contained in astrocytes. In white matter it is largely in the extracellular spaces17. Since the volume of the interstitial fluid in brain must necessarily depend on a balance of fluid exchanges across the cerebral capillaries and since the astrocytes are part of the blood-brain barrier, cerebral oedema must be related to a disturbance of fluid exchange at this interphase. An understanding of this disturbance presupposes a knowledge of the factors normally determining fluid distribution across the blood-brain barrier.