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A thin-film multiple-electrode probe for measuring DC potentials at eight sites with interdistances of 1 mm was constructed for the investigation of slow potential changes in deep regions of the human brain during surgery. The thin-film electrodes had to be placed on curved cylinder-shaped surgical instruments with dimensions of 2-mm diameter and 33-cm length used with the Freiburg stereotactic equipment. Several technological steps had to be introduced for solving the problems encountered. Structuring of the metal layers on curved substrates was accomplished by using flexible masks. Special feed-through technologies had to be invented in order to obtain reliable connections between the thin-film sensors and the copper wires inside the stereotactic instrument. Thin-film Ag-AgCl electrodes had to be formed in order to obtain satisfying recordings of slow potential changes below 10 Hz. Slow potential changes were recorded form different depths in interdistances of only 1 mm with the new miniaturized thin-film Ag-AgCl electrodes, and bipolar recordings with an electrode interdistance of only 3 mm clearly showed the appearance of Bereitschafts potentials.