Abstract： Objective To investigate the effect of long-term exposure to 0.1 mT 50 Hz magnetic fields (MF) on the ability of learning and memory, and the activities of superoxide dismutase (SOD), Na+- K+-ATPase, Ca2+-Mg2+-ATPase, and the malondialdehyde (MDA) level in hippocampus of old rats. Methods The changes in spatial learning and memory ability were examined by Morris water maze. The activities of SOD and ATPase and MDA level were determined by using commercially available kits. Results Acute ( 10 days) exposure to 0.1 mT 50 Hz MF did not affect the function of learning and memory in the old rats. However, after the old rats were exposed to 0.1 mT 50 Hz MF for 4 months or 8 months, the escape latency was obviously prolonged and the crossing annulus coefficient was reduced significantly, indicating that the spatial learning and memory function of old rats was damaged. Moreover, the activities of SOD, Na+-K+- ATPase and Ca2+-Mg2+ -ATPase were considerably inhibited, and the MDA level increased in hippocampus tissue and its mitochondria. Conclusion Long-term exposure (4 months or 8 months) to 0.1mT 50 Hz MF can damage the old rats' spatial leaning and memory function, which may be associated with the increase in lipid peroxidation in the hippocampus tissue.