Synapses are highly dynamic structures that mediate cellcell communication in the central nervous system. Their molecular composition is altered in an activity-dependent fashion, which modulates the efficacy of subsequent synaptic transmission events. Whereas activity-dependent trafficking of individual key synaptic proteins into and out of the synapse has been characterized previously, global activity-dependent changes in the synaptic proteome have not been studied.To test the feasibility of carrying out an unbiased largescale approach, we investigated alterations in the molecular composition of synaptic spines following mass stimulation of the central nervous system induced by pilocarpine. We observed widespread changes in relative synaptic abundances encompassing essentially all proteins, supporting the view that the molecular composition of the postsynaptic density is tightly regulated. In most cases, we observed that members of gene families displayed coordinate regulation even when they were not known to physically interact. Excitatory synaptic transmission is the primary mode of cell-cell communication in the central nervous system. The efficacy of synaptic transmission is highly regulated, and alterations in the strength of synaptic signaling within networks of neurons provide a mechanism for learning and memory storage, as well as for overall network stability. Modulation of synapse efficacy can occur through alterations in the structure and composition of the postsynaptic spine. The synaptic abundance of several molecules has been shown to be regulated in response to activity (1).The levels of individual proteins at postsynaptic spines are regulated through multiple processes. Active transport mechanisms exist and have been well characterized for AMPA-type glutamate receptors (AMPA-Rs) 1 via either insertion into the synapse or tighter association with the postsynaptic density (PSD) following lateral diffusion within the cell membrane (2). In addition to AMPA-Rs, other proteins known to be subject to activity-dependent regulation include calcium calmodulin-dependent protein kinase II alpha and beta, NMDA-type glutamate receptors (NMDA-Rs), and proteosome subunits (3-5). Synaptic protein content is dysregulated in a number of neuropsychiatric and neurodegenerative diseases, including Alzheimer's disease and fragile X mental retardation (6 -8).Most studies reported thus far have focused on a small number of selected molecules in individual experiments using a subset of synapses. Whereas learning and memory rely on the differential response of individual synapses to their specific input patterns, overall network excitability has to be maintained by homeostatic means. This homeostasis is governed by multiple pathways, and very little is known about the principles that regulate synaptic protein content across large numbers of synapses and neurons. The contributions of individual pathways and the interactions among them are largely unknown.In order to explore synaptic dynamics with a global view, we took adv...