Whole-cell patch-clamp recordings were obtained from light-responsive on-bipolar cells in retinal slices of the dogfish. Inclusion of the G-protein activator, GTP$\gamma $S, in the intracellular patch solution mimicked the action of glutamate, inducing an increase in net outward current (interpreted as a decrease in inward current), a decrease in membrane conductance and block of light responses. Cyclic GMP (cGMP) in the patch pipette increased inward current and membrane conductance, and blocked light responses. Cyclic AMP had no effect. IBMX, a phosphodiesterase inhibitor, produced the same effect as cGMP, suggesting the presence of a cGMP phosphodiesterase in rod bipolar cells. These results indicate that the glutamate receptors of on-bipolar cells are coupled via a G-protein to regulate intracellular cGMP, which, in turn, results in the opening of sub-synaptic membrane channels. The similarity to phototransduction is striking, and the proposed scheme would account for the high gain in transmission of rod signals to on-bipolar cells.