Extracellular single cell recording and microiontophoretic techniques were used to characterize the roles of ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs) in glutamate-induced excitation of rat nucleus accumbens (NAc) neurons in vivo. Pulse-ejected glutamate (16-128 nA) induced a current-dependent increase in the firing of quiescent NAc neurons. A stronger excitatory response to alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) was observed at much lower ejection currents (0.1-6.4 nA). Compared to AMPA and glutamate, N-methyl-D-aspartate (NMDA) induced a much less potent excitation in a narrow current range (1-4 nA) and only when neurons were previously "primed" with other excitatory amino acids (EAAs). Higher ejection currents of all three EAA agonists drove NAc neurons into a state of apparent depolarization block. AMPA-evoked firing was selectively blocked by the AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) whereas NMDA-induced activity was selectively prevented by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (D-AP5). DNQX, but not D-AP5, significantly attenuated glutamate-evoked activity. The mGluR receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-t-ACPD) failed to evoke activity of NAc neurons, but significantly reduced the excitatory effects of other EAAs. This modulatory effect of 1S,3R-t-ACPD was consistently blocked by the selective mGluR antagonist L(+)-2-amino-3-phopsonopropionic acid (L-AP3) whereas another mGluR antagonist (RS)-4-carboxy-3-hydroxy phenylglycine (4C3HPG) was inconsistent in this regard. These results indicate that the excitatory effects of glutamate on rat NAc neurons in vivo are primarily mediated by non-NMDA iGluRs and that mGluRs function to dampen excessive glutamate transmission through iGluRs.