Recent work has indicated that cobalt, at sub-millimolar concentrations, blocks horizontal cell (HC) to cone feedback, without attenuating direct cone to second-order cell synaptic transmission. We utilized low concentrations (0.25–0.5 mM) of cobalt to test the contribution of the feedback circuit, and other possible cobalt-sensitive mechanisms, to the receptive-field surrounds of retinal neurons. In the great majority of cases, low cobalt blocked ganglion cell surrounds, and it invariably blocked driving the ganglion cell by extrinsic current injected into the HC network. Although low cobalt reduced the integrating area of the HC network, dopamine, which similarly constricted the HC receptive area, did not block ganglion cell surrounds. Low cobalt reduced a late depolarizing wave in the HC light-evoked waveform and selectively suppressed the depolarizing component of chromatic HCs, both signs of HC to cone feedback. Low cobalt also reduced or blocked completely the receptive-field surrounds of a small sample of bipolar and amacrine cells. These results implicate the HC to cone feedback synapse in the formation of the receptive-field surround of retinal neurons.