Objective To investigate the effects of shortterm enriched environment on the hippocampal formation and the myelinated fibers in the hippocampal formation of midaged male rats. Methods Twenty 14month old male SD 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 spatial learning capacity of enriched rats and standard rats was tested with the Morris water maze. After the Morris water maze test, the total volume of the hippocampal formation and the myelinated nerve fibers in the hippocampal formation were quantitatively estimated with transmission electronic microscopy technique and stereological methods. Results There was not significant difference in the spatial learning capacity between enriched group and standard group. The total volume of the hippocampal formation of enriched rats was not significantly increased by 46% when compared with that of standard rats. The total volume, total length and mean diameter of the myelinated nerve fibers in the hippocampal formation of enriched rats were significantly increased when compared with those of standard rats. Conclusions Fourmonths enriched environment significantly affected the myelinated fibers in the hippocampal formation of midaged male SD rats. The present results might provide an important theoretical basis for searching the etho