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 female rats. Methods Twenty 14month female SD rats were randomly divided into 10 enriched environment (EE) rats and 10 standard environment (SE) rats. EE rats were reared in enriched environment and SE rats were reared in standard environment for 4 months. Then, five rats were randomly selected from each group. The spatial learning capacity was assessed with Morris water maze. The hippocampal formation and the myelinated fibers in the rat hippocampal formation were quantitatively investigated with transmission electronic microscopy technique and stereological methods. Results Short-term enriched environment enhanced the spatial learning capacity of the mid-aged female rats. The total length and total volume of the myelinated fibers in the hippocampal formation of the EE rats was significantly increased by 43.3% and 47.4%, respectively, when compared to the SE rats. There was no significant difference in the hippocampal volume and the mean diameter of the myelinated fibers in the hippocampal formation between two groups. The increase of the total length of the myelinated nerve fibers in the hippocampal formation was mainly due to the increase of the myelinated fibers with small diameter. Conclusion Short-term enriched environment had significant effects on the spatial learning capacity and the myelinated fibers in the hippocampal formation