Blood-brain barrier permeability for 24Na+ and 36Cl- ions and for 3HOH was studied during isoosmolality and during shortlasting intravascular hyperosmolality in twenty-five patients using the double indicatir single injection method. Hyperosmolality was induced by a rapid injection into the internal carotid artery of 8 ml of one of the following hypertonic solutions: 5% saline, 25% mannitol or a contrast medium of the metrizoat group (Isopaque-amine 280). The extractions of the small sodium and chloride ions remained unchanged and essentially zero indicating that 'opening' of the blood-brain barrier did not occur. The blood-brain barrier permeability to water remained unchanged during hyperosmolality, but the extraction of 3HOH increased when mannitol and especially when Isopaque-amine 280 was used as hypertonic agent corresponding to the lower water concentration in these solutions.