BACKGROUND: Mannitol is widely used in neurosurgical patients and may induce an increase in serum potassium concentration according to doses and administration rates with unknown mechanism. The treatment of hyperkalemia is aimed at eliminating the causes and includes calcium, sodium bicarbonate, glucose with insulin, loop diuretics and hyperventilation. This study was undertaken to observe the effects of hyperventilation on the serum potassium concentration following infusion of mannitol (2.0 gm/kg). METHODS: We studied 30 patients who were operated brain aneurysm clipping surgery and were divided into 3 groups (n=10). In control group, mild hypocapnia was maintained (PaCO2, 32 2 mmHg) before and after mannitol infusion. In group I, moderate hypocapnia was maintained (PaCO2, 27 2 mmHg) before and after mannitol infusion. In group II, mild hypocapnia (PaCO2, 32 2 mmHg) was maintained before 30 minutes of mannitol infusion and moderate hypocapnia (PaCO2, 27 2 mHg) after mannitol infusion. We started infusion of 20% mannitol with a dosage of 2.0 gm/kg, 15~20 min after cranium was opened. RESULTS: The changes of serum potassium were as follows (Mean SD mEq/l) (just before and 15min, 30min, 60min after mannitol infusion): 3.79 0.48, 4.66 0.60, 4.44 0.48, 4.13 0.40 (Control group), 3.62 0.18, 3.63 0.42, 4.14 0.51, 3.95 0.33 (Group I), 3.76 0.20, 3.91 0.15, 4.11 0.30, 4.04 0.23 (Group II). After 15 minutes of mannitol infusion, the serum potassium levels of group I and II were lower than that of control group (p<0.05) and there was no significant difference between group I and II. CONCLUSIONS: These results suggest that hyperventilation may blunt the increase in serum potassium concentration following rapid infusion of high dose mannitol.