1. Double-barrel, potassium-specific microelectrodes have been used to measure light-induced transient changes in [K+]o in the frog eye cup preparation. These changes in [K+]o have been termed the potassioretinogram (KRG). 2. The KRG consists of two components: a rapid increase in [K+]o in the proximal retina and a slow decrease in [K+]o in the distal retina. 3. The KRG decrease has the rhodopsin action spectrum, is maximal in the photoreceptor layer, persists after aspartate treatment, and has an increment threshold curve which saturates at moderate background intensities. The rhodopsin rods are, therefore, most likely the only neurons which generate this ionic change, although the Müller (glial) cells may also be involved in this process. 4. The KRG decrease has the same time course as the c-wave of the electroretinogram for all variations in the stimulus parameters, including intensity, duration, and chromaticity. 5. It is suggested that the c-wave may be produced by the pigment epithelial cells as they hyperpolarize in response to the decrease in [K+]o around the photoreceptors.