Electrical stimulation of the visual cortex causes subjects to see spots of light (phosphenes) in their visual fields. The authors have developed arrays of penetrating electrodes which can form the basis of a visual prosthesis centered around electrical stimulation of the visual cortex. These arrays have been constructed of inert materials, and have been implanted into cats to test array insertability and biocompatibility. Tests to determine electrode wire insertion force have also been performed. Preliminary acute histological results have shown little cortical damage due to the implants.