Brain edema is suggested to be the principal mechanism underlying the symptoms in acute hyponatremia. Identification of the mechanisms responsible for global and regional cerebral water homeostasis during hyponatremia is therefore of utmost importance. To examine the osmotic behavior of different brain regions and muscles, in vivo determined water content (WC) was related to plasma [Na(+)] and brain/muscle electrolyte content. Acute hyponatremia was induced with desmopressin acetate and infusion of a 2.5% glucose solution in anesthetized pigs. WC in different brain regions and skeletal muscle was estimated in vivo from magnetic resonance imaging (MRI) determined T(1) maps. WC, expressed in gram water per 100 gram dry weight, increased significantly in slices of the whole brain (342(SD=14) to 363(SD=21)) (6%), thalamus (277(SD=13) to 311(SD=24)) (12%), and white matter (219(SD=7) to 225(SD=5)) (3%). However, the WC increase in the whole brain and white mater WC was less than expected from perfect osmotic behavior, whereas in the thalamus, the water increase was as expected. Brain sodium content was significantly reduced. Muscle WC changed passively with plasma sodium concentration. WC determined with deuterium dilution and tissue lyophilzation correlated well with MRI-determined WC. In conclusion, acute hyponatremia induces brain and muscle edema. In the brain as a whole and in thalamus RVD is unlikely to occur. However, RVD may in part explain the observed lower WC in white matter. This may play a potential role in osmotic demyelination.