Intracerebral microdialysis was used to investigate the effects of local application of L-glutamate, N-methyl-D-aspartate, and the glutamate uptake inhibitor 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) in the nucleus accumbens (NAc) on extracellular dopamine (DA) concentrations. The effects of locally applied PDC on extracellular glutamate concentrations were also examined. Glutamate produced a concentration-dependent decrease in extracellular DA that could be blocked by concurrent, local application of the broad spectrum ionotropic glutamate receptor antagonist kynurenic acid (KYN:1 mM). N-Methyl-D-aspartate had a concentration-dependent effect on DA release, with a low concentration (0.1 mM) producing a decrease and a higher concentration (1.0 mM) resulting in an increase. Both effects were blocked by KYN. PDC (1 mM) increased extracellular glutamate concentrations to 102% above baseline. The same concentration of PDC decreased extracellular DA concentrations, and coapplication of KYN attenuated this effect. These results indicate that glutamate receptor agonists can have both facilitatory and inhibitory effects on extracellular DA concentrations. However, the effects of PDC indicate that inhibition of DA release is the more physiologically relevant effect. Furthermore, the results of these and other experiments suggest that glutamate's inhibitory effects on DA release in the NAc are not due to direct actions of this excitatory amino acid on DA terminals. A multisynaptic model that accounts for glutamate's actions on DA release is proposed.