Five patients, not related to each other, showed clinical signs, including electroretinograms, of a retinal dysfunction which mainly affected the cone system, but also involved the rod responses in a peculiar way. ERG b-wave threshold under dark adapted conditions was elevated. In contrast, rod b-wave sensitivity was enhanced with medium to high intensity flash stimulation. Furthermore, all patients had a severe reduction of the oscillatory potentials. The findings are discussed with special emphasis on a hypothetical disturbance in the cyclic guanosine monophosphate metabolism, involving both photoreceptors and cells of the inner plexiform layer responsible for retinal feedback mechanisms.