Objective To investigate the effects of exercise on the hippocampal formation and the myelinated nerve fibers in the hippocampal formation of middleaged rats. Methods Ten 14month female SD rats were randomly divided into exercise group and sedentary group.Rats in the exercise group were forced to run on a treadmill for 4 months. After 4 months, spatial learning capacity of two group rats was tested using the Morris water maze.Then, the hippocampal formation and the myelinated nerve fibers in the hippocampal formation were quantitatively estimated using transmission electronic microscopy and stereological techniques. Results Treadmill running enhanced the spatial learning capacity of the rats. The volume of hippocampal formation and the total length of the myelinated nerve fibers in the hippocampal formation were significantly increased after 4 months exercise.However,there was no significant difference in the total volume of the myelinated fibers in the hippocampal formation between the two groups.The absolute distributions of the total length of the myelinated fibers in the hippocampal formation of two groups indicated that the exerciseinduced 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. Conclusions Four months running exercise remarkably influence the spatial learning capacity,hippocampal formation and the myelinated fibers in the hippocampal formation of the middleaged famale SD rats. The present results reveal a potential mechanism for the fact that exercise