Current source-density analysis was used to study the spatial and temporal properties of b-wave current sources and sinks in the frog eye cup. The analysis shows that the b-wave arises primarily from a current source near the retinal surface and from two current sinks, one in the distal portion of the inner plexiform layer and the other in the outer plexiform layer. The magnitude of the distal (outer plexiform layer) sink peaks at an earlier time than does the proximal sink, and decays at a faster rate. At early times, however, the proximal sink is greater in magnitude. The results demonstrate that the currents of the proximal and distal sinks both contribute positively to the transretinal voltage, summing to give the b-wave response. The spatial and temporal properties of the two current sinks closely match those of the two light-evoked extracellular K+ ([K+]0) increases observed by others. This correspondence supports the hypothesis that increases in [K+]0 lead to the generation of b-wave currents. The distribution of these currents indicates that they flow through the glial cells of Muller.