The effects of local perfusion of the ventral tegmental area (VTA) with N-methyl-D-aspartic acid (NMDA) on extracellular dopamine concentrations in the nucleus accumbens were investigated by using in vivo microdialysis in halothane anaesthetized rats. The electrophysiological response of VTA dopamine neurons to NMDA were also assessed in an in vitro rat brain slice preparation. In both preparations NMDA elicited a biphasic response. Exposure of the VTA to low doses of NMDA (< 100 microM) elicited increases in dialysate dopamine levels in the nucleus accumbens and increases in the firing rate of VTA dopamine neurons. Larger doses (> 100 microM) resulted in profound reductions in both dopamine release in the accumbens and firing in the VTA. A strong correlation between the ability of NMDA to influence dopamine release in the accumbens and the firing rate in the VTA was observed. Perfusion with the non-competitive NMDA receptor antagonist PCP eliminated the NMDA-induced increases in extracellular dopamine in the accumbens. These data suggest that dopamine release in the accumbens and the firing rate of dopamine neurons can be both increased or decreased depending upon the magnitude of glutamatergic stimulation within the VTA.