Objective   To investigate the myelination of rat brain at different developmental stages.    Methods   Luxol fast blue staining

immunohistochemistry and Western blotting were applied to determine the distribution and alternation of myelin sheath in the brains of SD rats at different developmental stages.     Results   Negative LFB and MBP staining were shown at the stages of embryonicage 18 d (E18)

postnatal age 0 d (P0) and P2. At P7

the corpus callosum was weakly stained by LFB. The positive stain of LFB and MBP occurred in P15 on the corpus callosum. With the development going on

the stain turned out stronger

especially in P30

which was similar with that in P90 and P720. The results of Western blot analysis showed the expression of MBP in rat brain was gradually increased with the development of rat brain.     Conclusion   The myelination starts before P15 in rat brain and becomes mature in P30. The optimal observation time of myelin is 30 d born after.