Several lines of evidence suggest that, as concentrations of two agonists of group III metabotropic glutamate receptors are increased, cone contributions to the b-wave are blocked before rod contributions. Application of L-AP5 (L-2-amino-5-phosphonobutyric acid) at concentrations of 50 μM and D,L-AP4 (D,L-2-amino-4-phosphonobutyric acid) at concentrations 2 μM had a greater effect in reducing the amplitude of the rat ERG b-wave at high light intensities than at low light intensities. The amplitude reduction occurs at flash intensities that saturate rod photoreceptor responses. When steady backgrounds are used to saturate rod photoreceptors, the b-wave responses show increased long-wavelength sensitivity. Responses on a rod saturating background are blocked by adding L-AP5 or AP4 at the above concentrations to the perfusate. Further evidence for metabotrophic receptors being involved comes from the observation that even when ionotropic glutamate receptors are pharmacologically blocked with MK801 and DNQX, AP4 selectively blocks cone contributions to the b-wave. Thus we suggest that the type III metabotrophic receptors on depolarizing cone bipolar cells or cone synaptic terminals are affected by concentrations of L-AP5 and D,L-AP4 that have minimal effects on rod bipolar cells or rod synaptic terminals.