Objective  To explore the protective effects of hypoxia preconditioning on the injuries of hippocampal neurons and degression of memory ability caused by severe hypobaric hypoxia.     Methods    Wistar rats were divided randomly into control group (group C)

hypoxia group (group H

in hypobaric hypoxia chamber imitating 10 000 meters above sea level for 1 h/d

3 d) and hypoxia preconditioning plus subsequent hypoxia group (group HP

in hypobaric hypoxia chamber imitating 5 000 meters above sea level for 4 h/d

7 d

then treated as group H). The injuries and apoptosis of neurons in rat hippocampal CA1 were detected with HE staining

TUNEL and observed with electron microscopy. The changes of memory ability in group H and group HP after hypoxia were investigated with the active avoidance reaction.     Results    CA1 neuron injury in group HP was significantly slighter than group H

and apoptosis cells counting

electric shock times and mean time of active avoidance of rats in group HP were significantly lower than group H (P0.05).     Conclusion    Severe hypoxia induced the neuron apoptosis in hippocampal CA1 and lesions of memory in rats

while hypoxia preconditioning can attenuate these injuries efficiently.