Neuralink Corp. 1 article published in JoVE Neuroscience Chronic Implantation of Multiple Flexible Polymer Electrode Arrays Jason E Chung*1,2, Hannah R Joo*1,2, Clay N Smyth2, Jiang Lan Fan3, Charlotte Geaghan-Breiner2, Hexin Liang2, Daniel Fan Liu3, Demetris Roumis2, Supin Chen4,5, Kye Y Lee4, Jeanine A Pebbles4, Angela C Tooker4, Vanessa M Tolosa4,5, Loren M Frank2,6 1Medical Scientist Training Program and Neuroscience Graduate Program, University of California San Francisco, 2Kavli Institute for Fundamental Neuroscience, Center for Integrative Neuroscience, and Department of Physiology, University of California San Francisco, 3Bioengineering Graduate Program, University of California San Francisco, 4Center for Micro- and Nanotechnology, Lawrence Livermore National Laboratory, 5Neuralink Corp., 6Howard Hughes Medical Institute Described below is a method for implantation of multiple polymer electrode arrays across anatomically distant brain regions for chronic electrophysiological recording in freely moving rats. Preparation and surgical implantation are described in detail, with emphasis on design principles to guide adaptation of these methods for use in other species.