1. To explore the feasibility of a visual prosthesis for the blind, human visual cortex has been stimulated during a series of surgical procedures on conscious volunteers undergoing other occipital lobe surgery.2. Area no. 17 seems the most effective locus for such stimulation, at least in sighted or recently hemianopic patients.3. Changes in electrode size and configuration, or in stimulus parameters, have little effect on subjective sensation.4. Thresholds do vary depending on parameters, but not electrode size, and these effects have been studied.5. Painful effects are associated with stimulation of the dura, but not of the calcarine artery and associated vessels.6. Stimulation of a single electrode usually produces one phosphene, whose size ranges from tiny punctate sensations like ;a star in the sky' up to a large coin at arm's length. Very large elongated phosphenes, like those seen by Brindley's second patient, have not been reported despite the number of patients, electrodes, and combinations of stimulus parameters tested. These large phosphenes may be an effect of prolonged blindness.7. Stimulation substantially above threshold may produce a second conjugate phosphene, inverted about the horizontal meridian.8. Stimulation of a single electrode may also produce multiple phosphenes with no differential threshold.9. Chromatic effects and/or phosphene flicker may, or may not occur. This can vary from point to point on the same patient.10. Phosphenes fade after 10-15 sec of continuous stimulation.11. All phosphenes move proportionately with voluntary eye movements, within the accuracy of our mapping techniques.12. Brightness modulation can easily be achieved by changing pulse amplitude.13. The position of phosphenes in the visual field corresponds only roughly with expectations based on classical maps showing the projection of the visual field onto the cortex.14. Patients can usually discriminate phosphenes produced by 1 mm(2) electrodes on 3 mm centres, although this seems to be close to the limit of resolution.15. Patterns of up to four phosphenes produced by four electrodes have been recognized. However, a variety of complex interactions have been reported.16. Multiple phosphenes are co-planar, although patients are unable to estimate their distance.17. Phosphenes appear immediately when stimulation is begun, and disappear immediately upon cessation of stimulation.18. Future work must concentrate on blind volunteers to explore possible differences in subjective sensation produced after prolonged blindness, and to explore more complex pattern presentation which requires substantial periods of time with any given patient.