As in many central nervous system neurons, retinal ganglion cells (RGCs) receive fast synaptic activation through postsynaptic ionotropic receptors. However, the potential role of postsynaptic group I metabotropic glutamate receptors (mGluRs) in these neurons is unknown. In this study we first demonstrated that the selective group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine (DHPG), increased intracellular calcium concentration in neurons within the ganglion cell layer of the rat retina. This prompted us to use an immunopanned-RGC and cortical astroglia co-culture preparation to explore the effect of group I mGluR activation on the electrophysiological properties of cultured RGCs. Using perforated patch-clamp recordings in current-clamp configuration, we found that application of DHPG increased spontaneous spiking and depolarized the resting membrane potential of RGCs. This boosting effect was attributed to an increase in membrane resistance due to blockade of a background K⁺ conductance. Further experiments showed that the group I mGluR-sensitive K⁺ conductance was not blocked by 3 mM Cs⁺, but was sensitive to acidification. Pharmacological studies indicated that the effect of DHPG on RGCs was mediated by the mGluR1 rather than the mGluR5 receptor subtype. Our results suggest a facilitatory role for group I mGluR activation in modulating RGC excitability in the mammalian inner retina.