The achievement of long-term recording of neural signals from the central nervous system has potential clinical and investigative application. To facilitate long-term recording, a novel cone electrode composed of an insulated gold wire within a hollow glass cone had been developed. Cone electrodes containing sciatic nerve or neurotrophic medium were implanted into cerebral cortex in rats and monkeys. Electrophysiologic recordings had been previously obtained from cone tissue for as long as 15 months following implantation and this tissue contained silver-positive processes. We now extend these observations to characterize the fine structural features of the tissue within these long-term implants. Electron microscopy revealed central myelinated axons, dendrites, synaptic profiles, blood vessels, and glia; peripheral nerve was not found in the cones in which sciatic nerve had been placed. These observations further suggest ingrowth of cortical neurites and elements into the hollow glass tip of the cone and support the feasibility of long-term recording using this electrode.