By means of K⁺ microelectrodes, K⁺ potentials and [K⁺] were studied in frog retinas conditioned for SD by Cr-free Ringer's, A marked increase in [K⁺]₀ was observed during SD, the increase being maximal in the inner plexiform layer (up to 50mM or more) and subsiding towards the distal retina, The extracellular K⁺potential change during SD and also the graded K+potential change produced locally by iontophoretic injection of SD-stimulant chemicals resembled the membrane potential changes in Muller cells under the same conditions, suggesting that the Muller cells act as K⁺ electrodes, In retinas conditioned by C^--free Ringer's Müller cells were swollen, This allowed intracellular recording with K⁺electrodes in Müuller cells to reveal that upon SD the Müller cells immediately start to cleanse the extracellular space of excess ^K+ which is probably a product of pathologically enhanced synaptic activity in the inner plexiform layer, The mechanism of SDP, the neld potential change associated with SD, is discussed from a proposed model.