It is reasonably well known that there is a relationship between myelin formation and methylation of myelin basic protein in nerve cells. One of the suggestions is that arginine methylation of myelin basic protein could be of aid in the conjugation of myelin protein with the nonpolar lipid to form myelin. Abnormality in methylation of myclin basic protein might induce the neurological diseases in experimental animals as well as in human being. In the biological system, the methylation reaction is catalyzed by protein methaylse I using S-adenosyl-L-methionine as methyl donor. In this study, we examined the changes of S-adenosyl-L-methionine concentration and protein methylase I activity in developing rat brain tissues. The results are sumraerized as followings: (1) In brain tissues of fetus rat, the concentration of S-adenosyl-L-methionine was gradually decreased until to birth. However, the concentration in brain tissues of infant rat was suddenly increased at 7th day(just before myelination occur) birth. (2) Protein methylase I activity was decreased until to birth in brain of fetus rat and increased temporally just after birth, However, the enzyme activity showed no changes around 7th day after birth.