We have recorded from single neurons in the visual cortex of the cat while stimulating the cortical surface with the same type of electrodes and the same parameters of stimulation which have produced phosphenes in conscious man. Parameters of stimulation which will permit excitation of single cortical neurons with little risk of producing afterdischarges are described. The patterns of excitation of single neurons during surface stimulation and the effects of stimulation on neuroglial cells have also been studied. Surface stimulation also produced marked alterations in cortical excitability as tested with visually presented stimuli. Mechanisms by which single neurons are excited by surface stimulation are also considered.