Main olfactory bulb (MOB) granule cells (GCs) express high levels of the Group I metabotropic glutamate receptor (mGluR), mGluR5. We investigated the role of mGluRs in regulating GC activity in rodent MOB slices using whole-cell patch-clamp electrophysiology. The Group I/II mGluR agonist ACPD or the selective Group I agonist DHPG depolarized (~20 mV) and increased the firing rate of GCs. In the presence of ionotropic glutamate and GABA receptor antagonists, DHPG evoked a more modest depolarization (~8 mV). In voltage clamp, DHPG, but not Group II (DCG-IV) or Group III (L-AP4) mGluR agonists, induced an inward current. The inward current reversed polarity near the potassium equilibrium potential, suggesting mediation by closure of potassium channels. The DHPG-evoked inward current was unaffected by the mGluR1 antagonist LY367385, was blocked by the Group I/II mGluR antagonist LY341495, and was absent in GCs from mGluR5 knockout mice. LY341495 also attenuated mitral cell-evoked voltage-sensitive dye signals in the external plexiform layer and mitral cell-evoked spikes in GCs. These results suggest that activation of mGluR5 increases GC excitability, an effect that should increase GC-mediated GABAergic inhibition of mitral cells. In support of this: (i) DHPG increased the frequency of spontaneous GABAergic inhibitory postsynaptic currents in mitral cells, and (ii) LY341495 attenuated the feedback GABAergic postsynaptic potential elicited by intracellular depolarization of mitral cells. Our results suggest that activation of mGluR5 participates in feedforward and/or feedback inhibition at mitral cell to GC dendrodendritic synapses, possibly to modulate lateral inhibition and contrast in the MOB.