We studied the early and late effects of L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a competitive inhibitor of glutamate uptake with low affinity for glutamate receptors, in co-cultures of rat cortical neurons and glia expressing spontaneous excitatory amino acid (EAA) neurotransmission. At 100 or 200 μM, PDC induced different patterns of electrical changes: 100 μM prolonged tetrodotoxin-sensitive excitation triggered by synaptic glutamate release; 200 μM produced sustained, tetrodotoxin-insensitive and EAA-mediated neuronal depolarization, overwhelming synaptic activity. At 200 μM, but not at 100 μM, PDC caused rapid elevation of the glutamate concentration ([Glu]0) in the culture medium, resulting in NMDA receptor-mediated excitotoxic death of neurons 24 h later. The increase in [Glu]0 was largely insensitive to tetrodotoxin, independent of extracellular Ca2+, and present also in astrocyte-pure cultures. By the use of glutamate transporters functionally reconstituted in liposomes, we showed directly that PDC activates carrier-mediated release of glutamate via heteroexchange. Glutamate release and delayed neurotoxicity in our cultures were suppressed if PDC was applied in a Na+-free medium containing Li+. However, replacement of Na+ with choline instead of Li+ did not result in an identical effect, suggesting that Li+ does not act simply as an external Na+ substitute. In conclusion, our data indicate that alteration of glutamate transport by PDC has excitotoxic consequences and that active release of glutamate rather than just uptake inhibition is responsible for the generation of neuronal injury.