By means of K+ microelectrodes, K+ potentials and [K+] were studied in frog retinas conditioned for SD by CL--free Ringer's. A marked increase in [K+]o was observed during SD, the increase being maximal in the inner plexiform layer (up to 50 mM or more) and subsiding towards the distal retina. The extracellular K+ potential change during SD and also the graded K+ potential changes produced locally by iontophoretic injection of SD-stimulant chemicals resembled the membrane potential changes in Müller cells under the same conditions, suggesting that the Müller cells act as K+ electrodes. In retinas conditioned by Cl--free Ringer's Müller cells were swollen. This allowed intracellular recording with K+ electrodes in Müller 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 field potential change associated with SD, is discussed from a proposed model.