While using cobalt (Co2+) to block synaptic transmission in an in vitro preparation of chick retina, the authors observed significant changes in the DC electroretinogram (DC ERG). Cobalt (3.0 mM) increased the amplitudes of all three photoreceptor-dependent responses of the retinal pigment epithelium (RPE): the c-wave, fast-oscillation trough, and light peak. Intracellular recordings from RPE cells revealed that Co2+ increased those light-evoked changes in RPE membrane potentials that contribute to each of these responses. Monitoring of subretinal [K+]o with K(+)-selective microelectrodes showed that Co2+ increased the amplitude of the light-evoked [K+]o decrease, and this must contribute to the observed increase in c-wave and fast-oscillation trough because both are generated by this [K+]o change. Cobalt also increased the initial rate of [K+]o decrease at light-onset, the rate of subretinal [K+]o reaccumulation during maintained illumination, and the amplitude of the [K+]o overshoot at light-offset. The Co(2+)-induced increases in light-evoked RPE responses and subretinal [K+]o changes may arise from a direct effect on photoreceptors because (1) blockade of postphotoreceptoral activity did not block these effects of Co2+; and (2) Co2+ did not alter significantly the electrical properties of isolated RPE-choroid tissues. The authors conclude that, in the cone-dominated chick retina, Co2+ may act directly on the photoreceptors to increase the light response.