Illumination of the vertebrate retina evokes a transient decrease in extracellular potassium concentration, [K+]0, in the subretinal space. During maintained illumination, [K+]0 recovers toward its dark-adapted level. The mechanisms most likely to contribute to this recovery process were examined by using K+-selective microelectrodes to measure [K+]0 in the isolated retina of the toad. Bufo marinus. Although both diffusion of K+ and changes in the rod membrane voltage contribute to the recovery of [K+]0 during maintained illumination, other factors are likely to be involved as well. It is suggested that this recovery process could be due in part to inhibition of the Na+/K+ pump in the rod photo-receptors during maintained illumination.