Mannitol is an osmotic diuretic as a useful agent in the treatment of a variety of clnical conditions. In persons with normal renal function, the mannitol is almost excreted by kidney. Although various studies of the effectiveness of mannitol for the cerebral edema have been reported, but there are still few reports on acid-base disorder. This study is based on acid-base and electrolyte changes after the intravenous infusion of hypertonic mannitol for the purpose of preventing cerebral edema. Mannitol were intravenously infused with 300-900 mL for 90-minutes (group A), 1,200-2,600 mL for 24-hours(group B) and 3,200-4,900 mL for over 24-hours(group C) each. Each blood sample was drawn for gas analysis, and electrolytes through arterial line before and after mannitol infusion. In group A, blood pH is increased significantly from baseline level 7.43+/-0.07 to 7.46+/-0.04, and plasma HCO3- 25.3+/-2.1 mEq/L to 28.9+/-2.9 mEq/ L each, but plasma K is decreased significantly from baseline level 4.3+/-0.6 mEq/L to 3.7+/-0.8 mEq/L. In group B, blood pH is increased significantly from baseline level 7.42+/-0.02 to 7.47+/-0.06, and plasma HCO3- 25.2+/-1.8 mEq/L to 29.1+/-2.9 mEq/L each, but plasma K is decreased significantly from baseline level 4.2+/-0.3 mEq/L to 3.8+/-0.5 mEq/L. In group C, blood pH is increased significantly from baseline level 7.41+/-0.01 to 7.52+/-0.04, and plasma HCO3- 24.9+/-1.2 mEq/L to 27.7+/-2.5 mEq/L each, but plasma K is decreased significantly from baseline level 4.2+/-0.1 mEq/L to 3.9+/-0.2 mEq/L. These results clinically used intravenous infusion of mannitol could induce metabolic alkalosis associated with hypokalemia, regardless of its dosage.