The osmoregulatory system is well developed in the brain. Osmolytes contribute to maintenance of cell volume and cellular functions without changing intracellular ionic composition. Myo-inositol is regarded as one of the major osmolytes in the brain. In the present study, we investigated the changes in expressions of sodium myo-inositol cotransporter (SMIT) mRNA in the brain of acute hypernatremic rats by in-situ hybridization and Northern blot methods. Under moderate acute hypernatremic conditions, SMIT mRNA level increased markedly at 1 h and returned to almost control levels at 3 h, in accordance with plasma Na+ concentrations. Especially, distinct increases in SMIT mRNA expression were observed in the granule cells and glial cells in the cerebellum. These findings indicate that SMIT plays an important role in osmoregulation, especially in the early stages of acute hypernatremia in the brain.