Objective   To further observe the age-related changes in myelin sheaths of myelinated fibers in rat hippocampus.  Methods   Sixteen Long-Evans rats were divided in young age group (6-month old

n=5)

middle age group (18-month old

n=5) and old age group (27-month old

n=6). The mean diameter of myelinated fibers and axons

the inner and outer perimeter

and the mean thickness of myelin sheaths of myelinated fibers in rat hippocampus were measured with the new stereological methods and transmission electron microscope technique.  Results   The mean diameter of myelinated fibers in hippocampus was 10.8% higher in old age group than in middle age group（P0.05）but no significant difference was found between young and old age groups（P0.05）. The mean thickness of myelin sheaths of myelinated fibers with a diameter of less than 0.3 μm in hippocampus was 24.4% lower in old age group than in middle age group （P0.05） and was significantly higher in old age group with a myelin sheath diameter of 0.4

0.5

and 0.7 μm than in young age group（P0.05）.  Conclusion   Myelinated fibers with a small or a medium-sized diameter are demyelinated in hippocampus of normal old rats.