It is widely thought that persistent, use-dependent alterations in synaptic strength such as long-term synaptic potentiation (LTP) and depression (LTD) underlie at least a portion of memory traces in the brain, but the exact cellular locus of expression for these alterations remains to be determined. They could be expressed presynaptically as a decrease in transmitter release, postsynaptically as a decrease in the synaptic current evoked by a fixed delivery of transmitter, as an increase in the number of functional synaptic contacts, or by a combination of these mechanisms. Here we report that LTD at the climbing fiber-Purkinje cell synapse in rat cerebellum was not associated with changes in a synaptic cleft glutamate transient, indicating that this type of LTD is most likely expressed postsynaptically.