The metabolism of high grade astrocytoma was studied in 15 patients using intra-tumoural microdialysis. Two catheters were implanted during a stereotactic biopsy procedure: one in the tumour and one in the peri-tumoural tissue. The patients were fully mobilized the same day as the operation. Microdialysis samples were collected the next day and subsequently analysed for glucose, lactate, pyruvate, glutamate and glycerol. The main objective was to establish base-line levels of the studied substances. In addition, an in vitro experiment was performed in order to estimate recovery for the flow rates and catheters used. Glucose showed a tendency to be lower in tumour than in peri-tumoural tissue, indicating a high energy demand of the tumour. Lactate was significantly higher in tumour tissue. This supports previous reports that high grade astrocytomas use glycolysis rather than respiration to meet their energy demand. The tumours were also classified into necrotic and non-necrotic, according to the radiological finding. The necrotic tumours showed significantly higher levels of glutamate. They also presented a tendency to higher levels of glycerol than the non-necrotic tumours. These findings might be explained by the release of intracellular glutamate and of cell-membrane glycerol by cell destruction. We believe that microdialysis in awake and mobilized patients will prove to be a valuable tool in investigating metabolic events in malignant brain tumours especially during therapy.