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The entorhinal cortex is a key initial relay for cortical input to the hippocampus. To better understand hippocampal dysfunction resulting from early entorhinal cortex involvement in Alzheimer's disease, we stereotaxically injected ibotenic acid to produce unilateral entorhinal cortex lesions in rats. We then serially examined the CA3 hippocampal region by neuronal counts, histochemistry for acetylcholinesterase, and synaptophysin immunohistochemistry. Over 12 months, the neuronal counts did not change. Acetylcholinesterase-positive fibers were persistently but non-progressively beginning at 3 months. Synaptophysin immunoreactivity progressively declined over 12 months. Since much of the entorhinal cortex output proceeds to CA3 via the dentate gyrus, transsynaptic degeneration is suspected.