Objective To explore if there are significant differences in the white matter and the myelinated fibers between left and right hemispheres of male rats and if the age-related changes of the white matter and the myelinated fibers in the white matter are same for left hemisphere and right hemisphere of male rats. Methods The white matter and the myelinated fibers in the white matter from 5 male young Long-Evans rats (6-8 month old) and 4 male aged Long-Evans rats (18 month old) were quantitatively investigated with the stereological techniques and transmission electronic microscope technique. Results There are no significant differences of the white matter and the mylinated nerve fibers in the white matter between the left and right hemispheres in both young and aged groups. Although the total volume of the white matter and the total length and the total volume of the myelinated nerve fibers in the white matter were reduced in both left hemisphere and right hemisphere, only the white matter volume and the total volume of the myelinated fibers in the white matter of right hemisphere and the total length of the myelinated fibers in the white matter of left hemisphere were significantly decreased by 32.9%, 28.6% and 49.3%, respectively. Conclusion There are no significant differences in the white matter and myelinated nerve fiber in the white matter between left and right hemispheres in both young and aged male Long-Evans rats. The white matter volume and the total volume of the mylinaed fibers in the white matter of right hemisphere and the total length of the myelinated fibers in the white matter of left hemisphere in the aged male rats are significantly decreased when compared to young rats.