1. The present study examined the protective effects of icariin against the learning and memory deficits in aluminium-treated rats and its potential mechanisms of action. 2. Qualified rats were treated with 1600 p.p.m. AlCl(3) in drinking water for 8 months and the ability of spatial learning and memory was tested by the Morris water maze. In the place navigation test, aluminium administration significantly increased the mean escape latency and searching distance. In space probing test, aluminium markedly decreased the searching time and searching distance in the quadrant where the platform was originally located. All tests indicated deficits in rat spatial learning and memory induced by aluminium. Icariin treatment (60 and 120 mg/kg, by gavage for 3 months) dose-dependently protected against the development of aluminium-induced spatial learning and memory deficits. 3. To examine the mechanisms responsible for the protection afforded by icariin, the superoxide dismutase (SOD) activity and malondialdehyde (MDA) content in the hippocampus were assayed biochemically and the level of Abeta(1-40) in the hippocampus was determined immunohistochemically. Icariin treatment significantly increased SOD activity and decreased MDA and Abeta(1-40) content in the hippocampus of aluminium-intoxicated rats. 4. In conclusion, the present study demonstrates that icariin is effective in improving the spatial learning and memory of aluminium-intoxicated rats. The mechanisms responsible appear to be due, at least in part, to an increased anti-oxidant capacity and decreased lipid peroxidation and Abeta(1-40) levels in the rat hippocampus.