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Long-term potentiation (LTP), an increase in synaptic efficacy following high-frequencystimulation, is widely considered a mechanism of learning. LTP involves local remodeling ofdendritic spines and synapses. Smooth endoplasmic reticulum (SER) and endosomal compartmentscould provide local stores of membrane and proteins, bypassing the distant Golgi apparatus. Totest this hypothesis, effects of LTP were compared to control stimulation in rat hippocampal areaCA1 at postnatal day 15 (P15). By two hours, small spines lacking SER increased after LTP, whereaslarge spines did not change in frequency, size, or SER content. Total SER volume decreased afterLTP consistent with transfer of membrane to the added spines. Shaft SER remained more abundantin spiny than aspiny dendritic regions, apparently supporting the added spines. Recyclingendosomes were elevated specifically in small spines after LTP. These findings suggest localsecretory trafficking contributes to LTP-induced synaptogenesis and primes the new spines forfuture plasticity.