THE PRESSURE of the cerebrospinal fluid is dependent upon the volumes of blood, cerebrospinal fluid, extracellular fluid, and intracellular fluid within the cranial cavity. The alteration in cerebrospinal fluid pressure resulting from expansion of the extracellular fluid volume, as opposed to an expansion of the intracellular fluid volume, is of theoretical interest and may be of practical importance in the management of patients with increased intracranial pressure. The relative volumes of the extracellular and intracellular fluids can be experimentally changed by taking advantage of the different volumes of distribution of two isotonic solutions, 0.9% saline and 5% dextrose in water, following intravenous administration. The rapid intravenous administration of either solution causes an initial expansion of the plasma volume. The further distribution of 0.9% saline is essentially limited to the extracellular fluid volume, while 5% dextrose in water is distributed to both the extracellular and the intracellular fluid volume. Thus, saline