ABSTRACT Changes in sensitivity of the photoreceptor cells of the anterior median eye of the jumping spider Menemerus confusus Boes, et Str. have been studied by recording electroretinograms (ERGs) and receptor potentials. The amplitudes of the responses (ERGs and receptor potentials) increase during repetitive stimulation, with a maximum increase at 3−5 s intervals. The sensitivity of the photoreceptor cell is greater for about 60 s following illumination (maximum magnitude at 3−5 s) than it is during complete dark adaptation. This phenomenon, which we call ‘hypersensitivity’, is lost within one day following surgery in physiological saline. Upon loss of hypersensitivity, the sensitivity decrease during light adaptation is greater than for the normal eye and the small increase of sensitivity following the onset of illumination observed for the normal eye is lost.