The infusion of isoosmolar glycerol (0.35 mol/dm-3) into the ventricles of laboratory rats for 120 minutes led to an increase in the serum osmolality by 11 mosm/kg and to hypernatremia. The brain water content of the cerebral hemispheres decreased by 0.9% (P less than 0.05). A corresponding intraventricular infusion of saline or d-glucose did not cause significant changes in these parameters. These findings support the view that glycerol, even in a dose incapable of creating a major osmotic gradient between plasma and brain, could have a beneficial effect in the control of intracranial volume-pressure perturbations. It is hypothesized that, besides acting as an osmotic dehydrating agent in certain concentrations, glycerol influences the central neuroendocrine system responsible for brain ion and volume homeostasis. By its presumed reduction of central and peripheral vasopressin release through lowering the cerebrospinal fluid sodium concentration, it may help in decreasing the brain water content.