Until very recently, edema of the brain was generally thought to resemble that of other organs in consisting of the accumulation of fluid in an extracellular space, in addition to any intracytoplasmic changes. Recent electron microscopic studies have been interpreted as indicating that such extracellular space is virtually nonexistent, except possibly for a relatively constant zone no larger than 200 A. between adjacent cellular processes.1-5In edematous brains, electron microscopic studies disclosed swelling of a cellular process with a pale, clear cytoplasm.6-9These pale cellular processes have been interpreted as those of oligodendroglial cells,1,9,10as those of astrocytes,2-6or have not been otherwise identified.7,8 *There is no disagreement, however, that it was these clear glial processes which were swollen in edema, and that there was no accumulation of fluid in an extracellular space. In the light of these observations it was thought wise to review