The level of synapsin I, a synaptic vesicle protein and marker for synaptic activation, was studied 8 h after long-term potentiation (LTP) induction in rat hippocampus in vivo. Quantitative immunohistochemical analysis revealed that synapsin I increased significantly in the molecular layer of the dentate gyrus and the stratum lucidum of CA3, suggesting activation of the granule cells by synaptic input to the dentate gyrus, and persistent enhancement of glutamate release from the axon terminals of the dentate granule cells.