Chronically implanted microelectrodes have been an important tool used by neuroscientists for many years and are critical for the development of neural prostheses designed to restore function after traumatic central nervous system (CNS) injury. It is well established that a variety of mammals, including non-human primates (NHP), tolerate noble metal electrodes in the cortex for extended periods of time, but little is known about the long-term effects of electrode implantation at the cellular level. While data from rodents have clearly shown gliosis around such implants, there have been difficulties in demonstrating these reactions in NHP. Glial reactions are to be expected in NHP, since any trauma to the mammalian CNS is believed to result in the formation of a glial scar consisting of reactive astrocytes and microglia around the injury site. Because a glial scar can potentially affect the quality of recordings or stimulations from implanted electrodes, it is important to determine the extent of gliosis around implants in NHP. We studied the response of cortical glial cells to chronic electrode implantation in the motor cortices of Rhesus macaques (Macaca mulatta) after 3 months and 3 years duration. Antibodies specific for astrocytes and microglia were used to detect the presence of glial reactions around electrode implant sites. Reactive glia were found within the cortical neuropil surrounding the chronically implanted noble metal electrodes. Reactive gliosis persisted over the time periods studied and demonstrates the importance of developing strategies to minimize this event, even around noble metal implants.