1. We have used patch-clamp recording techniques to study the physiological properties of a recently described glutamate uptake blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), in the CA1 region of the guinea pig hippocampus. 2. L-trans-PDC markedly potentiated the action of exogenously applied glutamate and raised the ambient extracellular levels of glutamate in hippocampal slices. Despite these actions, L-trans-PDC did not affect the time course of either the N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated synaptic currents evoked by the stimulation of a large number of neighboring synapses. 3. These findings are consistent with models of fast synaptic transmission in which transmitter is rapidly cleared from the synaptic cleft by diffusion. However, in marked contrast to fast gamma-aminobutyric acid A (GABAA) synapses in the hippocampus, uptake does not appear to play a role in regulating the "spill-over" of transmitter from neighboring, co-activated glutamatergic synapses. Therefore, either diffusion alone can effectively limit the temporal and spatial domain of synaptically released glutamate, or alternatively, L-trans-PDC like other currently available blockers is not sufficiently potent to reveal a role for transmitter uptake at glutamatergic synapses.