The fluid compartments within the central nervous system are unique in that virtually no extracellular space exists. Membrane touches membrane in the closely packed neuropil and even the capillaries are closely invested by glial expansions without an intervening space excepting that occupied by the endothelial basement membrane. Since electron micrographs have consistently demonstrated the paucity of the extracellular space in brain and spinal cord,1-3the question has arisen as to exactly what compartment may be equated with the measurable chloride space. It has been suggested that this space may be equivalent to the oligodendroglial cytoplasm, and furthermore that the oligodendroglia may serve as the main route of water transport within the central nervous system.4-6 In experimentally induced swelling of the brain the increased fluid is within the oligodendroglial cytoplasm and its extensions, not in an extracellular space. This has been demonstrated by electron microscopy on specimens after swelling