Synaptic transmission at excitatory synapses can be regulated by changing the number of synaptic glutamate receptors (GluRs) through endocytosis and exocytosis. The endocytosis of GluRs has recently been shown to require the activity of the ubiquitin-proteasome system (UPS): proteasome inhibitors or dominant negative forms of ubiquitin block the ligand-stimulated internalization of GluRs. We have examined whether PSD-95 is a potential target of the UPS. Following neurotransmitter stimulation, PSD-95 levels are negatively correlated with the magnitude of internalized GluR1 in individual neurons. Neurotransmitter stimulation also results in a proteasome-dependent decrease in dendritic PSD-95. Consistent with the idea that PSD-95 degradation is important for GluR internalization, overexpression of PSD-95 can inhibit neurotransmitter-stimulated GluR1 endocytosis. If PSD-95 is a direct target for proteasomal degradation, then the polyubiquitination of PSD-95 is expected. Using experimental conditions that favor the detection of polyubiquitination, however, no ubiquitination of PSD-95 was detected. It is possible that the polyubiquitination of PSD-95 is short-lived and thus difficult to detect. Alternatively, the regulation of PSD-95 levels by the proteasome important for ligand-stimulated GluR endocytosis may be accomplished via an intermediate protein.