An increase in cerebrospinal fluid pressure incurs in dogs and humans anesthetized with nitrous oxide after air has been injected into the cerebral ventricles. The mechanism for the increase in pressure is felt to be due to the difference in blood solubility between nitrous oxide and nitrogen. Nitrous oxide, being 30 times more soluble in blood than nitrogen, is carried to the air-containing ventricles in a greater quantity than the amount of nitrogen that can be carried away from the ventricles. The result is an increased number of molecules within the ventricles and an increase in intraventricular pressure. This pressure increase might prove fatal, especially in an individual whose cerebrospinal fluid pressure is already elevated. Either the avoidance of nitrous oxide anesthesia or the use of nitrous oxide as the contrast gas would eliminate the rise in pressure.