Activity-regulated, cytoskeletal-associated protein (Arc) is an immediate early gene induced in excitatory circuits following behavioral episodes. Arc mRNA is targeted to activated regions of the dendrite after long-term potentiation (LTP) of the dentate gyrus, a process dependent on NMDA receptor activation. We used post-embedding immunogold electron microscopy (EM) to test whether synaptic Arc expression patterns are selectively modified by plasticity. Consistent with previous light microscopic observations, Arc protein was rapidly induced in the dentate gyrus following LTP-producing stimulation of the perforant path and was detectable in granule cell nuclei, somata and dendrites after two hours of high frequency stimulation. Post-embedding EM revealed Arc immunogold labeling in three times as many spines in the middle molecular layer of the stimulated dentate gyrus than in either the ipsilateral outer molecular layer or the contralateral middle and outer molecular layers. This upregulation did not occur with low frequency stimulation of the perforant path. Therefore Arc protein localization may be a powerful tool to isolate recently activated dendritic spines.