We examined whether the critical protein synthesis for maintenance of perforant path long-term potentiation (LTP) takes place in the dentate gyrus or the entorhinal cortex. Field potential recordings were made of responses in the dentate gyrus to stimulation of the perforant path in urethane-anaesthetized rats. Anisomycin (10 micrograms) injected into the dentate gyrus, but not the entorhinal cortex, 1 h prior to tetanization led to nearly complete decay of perforant path LTP of the excitatory postsynaptic potential (EPSP) within 3 h. Intra-dentate injection of neither actinomycin D (a mRNA synthesis inhibitor) nor boiled anisomycin affected LTP maintenance over 6 h. These results suggest that the proteins necessary for the maintenance of LTP over 6 h are synthesized in the dentate gyrus from already existing mRNA without involving protein synthesis in the cell bodies of the afferent fibres.