We experienced an initial case in which the recordings of glutamate level were continuously followed by microdialysis just immediately after ictus. A 51-year-old woman, who had a subarachnoid hemorrhage on computed tomography, was comatose on admission. After ventricle drainage, a microdialysis catheter was inserted using the same drill hole that had been used for ventriculostomy. When systemic blood pressure and intracranial pressure (ICP) suddenly increased 7.5 hours after admission, the glutamate levels from the right frontal lobe were maximally 388.5μmol·l^-1 at that time. Although her pupils and ICP recovered to normal within 1.5 hours, the glutamate level took more than 6 hours to return the normal range. Thereafter, systemic blood pressure and ICP repeated sudden shoot-ups, and the glutamate level was extraordinary high (>400μmol·l^-1) throughout the period. Finally ICP got uncontrollable and we missed this patient. We need further study about the usefulness of the measurement of glutamate using microdialysis in patients with poor grade subarachnoid hemorrhage.