Surgical technic where we insert a canula attached to a membrane inside the brain. A physiological liquid is circulating trough pumping and, because membrane pores, small molecules are collected in tubes. Brain microdialysis is used to sample and collect substances from the interstitial space, including neurotransmitters, toxins and electrolytes. It enables the monitoring of in vivo changes in the interstitial composition throughout the time and after experimental manipulations. The microdialysis probe (catheter) is inserted in specific brain regions. Such probe acts as a semi-permeable membrane that allows the transfer of solutes depending on their concentration gradients. The probe is continuously perfused with an aqueous solution (perfusate) which constitution’s has a similar ionic concentration to that of the interstitial fluid. The probe is connected to sophisticated analyzers such as HPLC, therefore the sample (dialysate) can be analyzed.