Electroretinogram (ERG) and extracellular potassium activity (K0+) measurements were carried out in isolated superfused rabbit eyecup preparations under control conditions and during the application of pharmacological agents that selectively modify the light-responsive retinal network. Light-evoked K0+ changes in the rabbit (E-type) retina resemble those previously described in amphibian (I-type) retinas. Different components of the light-evoked K0+ changes can be distinguished on the bases of retinal depth, V vs. log I properties, and their responses to pharmacological agents. We find two separable sources of light-evoked increases in extracellular K+: a proximal source and a distal source. The properties of the distal light-evoked K0+ increase are consistent with the hypothesis that it initiates a K+-mediated current through Müller cells that is detected as the primary voltage of the electroretinographic b-wave. These experiments also support previous studies indicating that both the corneal-positive component of c-wave and the corneal-negative slow PIII potential result from K+-mediated influences on, respectively, the retinal pigment epithelium and Müller cells.