Objective   To investigate the sex differences of the myelinated nerve fibers in the hippocampal formation of mid-aged standard and enriched rats.    Methods   Twenty 14-month Sprague-Dawley (SD) rats

10 female and 10 male rats

were randomly divided into enriched group and standard group. Enriched rats were reared in enriched environment and standard rats were reared in standard environment for 4 months. Then

the total volume of the hippocampal formation and the myelinated nerve fibers in the hippocampus were quantitatively estimated by transmission electron microscopy and stereological methods.    Results   The total volume and mean diameter of the myelinated nerve fibers in the hippocampal formation of standard female rats were significantly bigger than those of standard male rats (P0.05)

but there was no statistically significant change of the total volume of the hippocampal formation and the total length of the myelinated nerve fibers in the hippocampal formation of standard female rats when compared to those of standard male rats (P0.05). The mean diameter of the myelinated nerve fibers in the hippocampal formation of enriched female rats was significantly smaller than that of enriched male rats (P0.05)

but there was no statistically significant difference of the total volume of the hippocampal formation and the total volume and total length of the myelinated nerve fibers in the hippocampal formation of enriched female rats when compared to those of enriched male rats (P0.05).    Conclusion   The sex differences of  the myelinated nerve fibers in the hippocampal formation of mid-aged standard and enriched rats are significantly different

which indicates the response to the enriched environment in the mid-aged male and female rats is different.