Résumé : Preliminary evidence has suggested that magnesium sulfate infusion reduces delayed ischemic neurological deficit and improves clinical outcome after aneurysmal subarachnoid hemorrhage. However, little is known about its site of action in vivo. We studied 22 aneurysmal subarachnoid hemorrhage patients with or without magnesium sulfate infusion for 10-14 days. Thirteen patients had external ventricular drains inserted for hydrocephalus and daily cerebrospinal fluid magnesium levels were measured. For patients given magnesium sulfate infusion, the aim was to raise the plasma magnesium level to double the baseline level. We found that the magnesium sulfate infusion bought an 11% to 21% increase in cerebrospinal fluid magnesium. The elevation of cerebrospinal fluid magnesium was sustained for at least nine days. Whether this mild elevation in cerebrospinal fluid magnesium level was adequate for neuroprotection awaits the results of ongoing clinical trials.