ON bipolar neurons in retina detect the glutamate released by rods and cones via metabotropic glutamate receptor 6 (mGluR6), whose cascade is unknown. The trimeric G-protein G_o might mediate this cascade because it colocalizes with mGluR6. To test this, we studied the retina in mice negative for the α subunit of G_o (Gα_o−/−). Retinal layering, key cell types, synaptic structure, and mGluR6 expression were all normal, as was the a-wave of the electroretinogram, which represents the rod and cone photocurrents. However, the b-wave of the electroretinogram, both rod- and cone-driven components, was entirely missing. Because the b-wave represents the massed response of ON bipolar cells, its loss in the Gα_o null mouse establishes that the light response of the ON bipolar cell requires G_o. This represents the first function to be defined in vivo for the α subunit of the most abundant G-protein of the brain .