THE WIDESPREAD clinical use of hypertonic urea solutions to reduce intracranial pressure has stimulated a number of studies on the mechanism of its action1-8and a certain degree of controversy has arisen as to whether a rebound increase in the pressure occurs.1,9Our findings, that in traumatically induced cerebral edema accumulation of fluid occurs mostly in the white matter,10brought up the questions whether the dehydrating action of urea affects all brain tissue or is restricted to one type of tissue and to what extent infusion of hypertonic urea solution can reduce the accumulation of edema fluid in white matter. The present paper deals with experiments designed to study the effect of hypertonic urea on the water, sodium, potassium, and urea content of white matter and cerebral cortex tissue of normal cats and of cats in which edema had been induced by freezing lesions.