Photoreceptor loss due to severe retinitis pigmentosa or macular degeneration is one of the major causes of blindness in the Western world,and the prevalence of these diseases is likely to increase dramatically as the population ages. Currently there are several groups trying to develop retinal prostheses, analogous to cochlear implants, in which photoreceptor input is replaced by direct electrical stimulation. Recently six patients have been implanted chronically with simple 4x4 retinal prostheses lying over the inner retinal layer. We report here psychophysical data examining the perceptual consequences of electrical stimulation on a single electrode. These data include (1) how thresholds decrease with pulse duration and decrease as a function of electrode height above the retinal surface, (2) how brightness increases as a function of stimulation intensity, and (3) how pulses interact over time.