Chronic recording intracortical microelectrodes (c.r.i.m.s) maintain connections between external devices and single neural elements in the cerebral cortices of unrestrained animals. Such electrodes are tethered to the skull by means of an ultraflexible lead and cemented to the surface of the brain by a drop of biodegradable adhesive, and they are contained within a fluid-filled chamber which re-establishes the integrity of the cranial vault. The chronic electrode described offers the advantages of reproducibility and mass manufacture. A new combination of biomaterials (pure iridium for the shaft, pure gold for the flexible lead and poly-monochloro-p-xylene as the insulator) meets those mechanical constraints which provide for stability of electrode tip position. Design, fabrication andin vivo behaviour of the electrode are described in detail. A description is included of a technique by which the position of the c.r.i.m. may be adjusted while in the animal.