(-)-Huperzine A-d5 (Major) is the labelled form of (-)-Huperzine A (H826000), a reversible alkaloid inhibitor of AChE which crosses the blood-brain barrier. A potential therapeutic agent for Alzheimer's disease. It reduces cell death induced by glutamate in primary cultures derived from forebrain, hippocampus, cortex and cerebellum of embryonic rat brain.