In 12 patients with increased intracranial pressure, caused by an expanding process, a hypertonic urea solution was intravenously administered during a craniotomy. At different times before, during and after the operation, the electrolytes, urea, glucose and total protein values were determined in various body fluids and tissues. This study disclosed that the urea administered was distributed through both the intracellular and the extracellular space after 20 minutes. The values of the electrolytes, except the calcium, in the extracellular fluid remained constant after administration of the urea solution; the total protein value, however, showed a considerable decrease. It was established that the blood-brain barrier plays no appreciable role in the mechanism of action of hypertonic urea solutions in dehydrating the brain tissue; the blood-C. S. P. and brain-C. S. F. barriers may do.