Objective        To determine the effects of estrogen replacement therapy (ERT) on the myelinated fibers in CA1 of the hippocampus in middle-old ovariectomized (OVX) rats.       Methods       Female rats of 10 months old were bilaterally ovariectomized

and then randomly divided into OVX+E2 group and OVX+Veh group. In 4 weeks after operation

17β-estradiol (0.1 mg/mL

0.25 μL/g) and placebo (sesame oil

0.25 μL/g) were subcutaneously injected respectively to OVX+E2 rats and

OVX+Veh rats for 4 weeks. Then

  Morris water maze was applied to test the spatial reference memory. Microscopy and stereology were applied to quantitatively investigate the total volumes of the hippocampus

the CA1 of the hippocampus

the total length

total volume

and mean diameter of the myelinated fibers and the myelin sheaths in the hippocampal CA1.       Results       The mean escape latency in spatial navigation of Morris water maze was significantly shortened in OVX+E2 group from day 1 to 5 when compared to OVX+Veh group （

P

0.05）. There was no significant difference in the hippocampal volume and CA1 volume between 2 groups (

P

=0.100

P

=0.366). When compared to OVX+Veh group

the total volume of the myelinated fibers and myelin sheaths in OVX+E2 group were significantly increased (

P

=0.016

P

=0.009).       Conclusion       Continuous 4 weeks’ ERT protects the spatial memory and the structure of myelinated fibers and myelin sheaths in the hippocampal CA1 of middle-aged ovariectomized rats.