Objective：To investigate the effects of short-term estrogen replacement on myelinated nerve fibers and myelin sheaths in CA1 region of ovariectomized rats. Methods：Female SD rats（10-11 month-old） were bilaterally ovariectomized. One month after o－variectomy，successful ovariectomized rats were randomly divided into placebo group（OVX+O） which received sesame oil for 4 weeks and estrogen replacement group（OVX+E） which received continuous 17β-estradiol replacement treatment for 4 weeks. After 4 weeks，spatial learning capacities of two groups were tested using the Morris water maze. Then，the serum estradiol levels were detected by chemiluminescence method. The myelinated nerve fibers and myelin sheaths in CA1 region were quantitatively investigated by transmission electron microscope and stereological methods. Results：The serum estradiol levels were significantly increased after es－trogen replacement treatment（t=5.688，P=0.000）. The spatial learning capacity of OVX+E rats was significantly improved when com－pared with that of OVX+O rats（F=9.645，P=0.006）. There was no significant difference in the CA1 volume，the volume of myelinated nerve fibers，the volume of myelin sheaths，the mean diameter of the myelinated fibers and the mean thickness of myelin sheaths be－tween two groups（P=0.181，P=0.294，P=0.521，P=0.545，P=0.081，respectively）. The mean thickness of myelin sheaths of the myeli－nated fibers with diameters between 0.6 to 0.7 μm was significantly higher by 14.1% in OVX+E group when compared with that of OVX+O group（t=2.428，P=0.038）. The difference between the mean diameter of myelinated nerve fibers and the mean diameter of axons with the mean diameter of 0.8 to 0.9 μm in OVX+E group was significantly increased by 18.2% when compared with that of OVX+O group（t=2.432，P=0.038）. Conclusions：Short-term estrogen replacement can enhance the spatial learning capacity and can exert pro－tective effects on the myelin sheaths in the CA1 region of middle-aged ovariectomized rats.