Brain microdialysis, performed in freely moving animals, is based on the diffusion of molecules via a semi-permeable membrane from the extracellular fluids to the physiological salt solution perfusing the microdialysis probe. The content of, for instance, dopamine in the perfusate is measured by normal-bore and capillary High Performance Liquid Chromatography (HPLC). It reflects the release of dopamine from nerve terminals in the brain tissue around the semi-permeable wall of the microdialysis probe. We can measure dopamine with high temporal resolution, enabling us to correlate the activity of the dopaminergic system with behavioral events.