Microdialysis in the human brain has been performed for the first time during thalamotomy intended to relieve tremor in patients with Parkinson's disease. The aim was to test the reliability of the microdialysis technique for biochemical characterization of a target area in the human brain during a routine operation. Microdialysis probes were introduced through the same trajectory as the lesioning electrode thus causing no additional damage to the brain. Dopamine, DOPAC, HVA, 5-HIAA, hypoxanthine, inosine, guanosine, adenosine, GABA, taurine, aspartate and glutamate were measured in the perfusate from the target region - the Vim nucleus. The results show initial high levels that reach baseline levels after 10-20 minutes. Surprisingly, consistent and reproducible levels were found, the only exception being one patient on 1-DOPA therapy who had elevated DA and metabolite levels.