After the oral administration of 50% glycerol (12 mL/kg), serum, CSF and inner ear fluids from scala tympani perilymph, scala vestibuli perilymph, and scala media endolymph were collected from normal guinea pigs under sodium pentobarbital anesthesia (25-35 mg/kg). The sodium and potassium concentrations were determined by microflame photometry. Increases in sodium concentrations were found in CSF, scala tympani perilymph, scala vestibuli perilymph, and cochlear endolymph. No significant change was observed in the serum. These sodium increases were considered to be due to the dehydration caused by the osmotic action of glycerol. Potassium concentration was increased only in scala tympani perilymph. Oral administration of glycerol was found to be more gradual and effective in dehydration compared to intravenous injection.