Abstract: The effccts of mifepristone on the cerebral ultrastructure of term pregnant rats were studied. Ten pregnant rats were divided randomly into an experimental group （n = 5） and a control group （n = 5） . At the 20th day of gestation, the experimental rats were given sesame oil 1 ml and mifespristone 11.5±1mg, while the control rats were-given only sesame oil 1 ml. After labour, the cortical tissues of the right parietal cere-brum were obtained and prepared for electron microscopic observation and analysed by stereology. Results ：compared with the control group, the experimental group showed the following morphological changes： （1） Neurons swelled and membranous cytoplos-miz organoids decreased ； （2） Neuropil was partially damaged and major dense lines of myelin sheath were separated； （3） Synaptic connections decreased； （4） Interstitium was swollen ； （5） The numerical density of mitochondria in the experiment group was de-crease,but the volume density showed no significant change. Conclusion ： Mifespristone could cause ischemia-hypoxia changes in cerebrum of pregnant rats.

Key words: Mifepristone , Rat , Cerebrum , Ultrastructure , Stereology