Mannitol is an osmotic diuretic agent useful in a variety of clinical conditions. This study is based on acid-base and electrolyte changes seen after the intravenous infusion of hypertonic mannitol for the prevention of cerebral edema. The study subjects were divided into 3 groups: for group A, an amount of 300-900 mL 15% mannitol was intravenously infused over the period of 60 to 90 minutes; for group B, 1,200-2,600 mL over 12 to 24 hours; and for group C, 3,200-4,900 mL over more than 24 hours. In group A, blood pH is increased from 7.43+/-0.07 to 7.46+/-0.04, and plasma HCO3- from 25.3+/-2.1 to 28.9+/-2.9 mEq/L, but plasma K+ is decreased from 4.3+/-0.6 to 3.7+/-0.8 mEq/L. In group B, blood pH is increased from 7.42+/-0.02 to 7.47+/-0.06, and plasma HCO3- from 25.2+/-1.8 to 29.1+/-2.9 mEq/L, but plasma K+ is decreased from 4.2+/-0.3 to 3.8+/-0.5 mEq/L. In group C, blood pH is increased from 7.41+/-0.01 to 7.52+/-0.04, and plasma HCO3- from 24.9+/-1.2 to 27.7+/-2.5 mEq/L, but plasma K+ is decreased from 4.2+/-0.1 to 3.9+/-0.2 mEq/L. These results showed that intravenous infusion of mannitol could induce metabolic alkalosis and hypokalemia, regardless of its dose. The mannitol induced metabolic alkalosis may be due to increased renal HCO3- production.