β 1 , 4 -Galactosyltransferase (GalTase, EC 2.4.1.38) transfers galactose to the terminal N-acetylglucosamine of complex-type N-glycans, which have great importance for cell-cell interactions during fertilization and early embryogenesis. In this study, the activity of β 1 , 4 -galactosyltransferase in mouse brain during development was measured with the method of reverse HPLC using a fluorescence-labeled biantenary sugar chain, GlcNAcβ1-2Manα1-6(GlcNAcβ1-2Manα1-3) Manβ1-4GlcNAcβ1-4GlcNAc-PA. The level of messenger RNA of this enzyme during the development of mouse brain was also investigated with Northern blot analysis. The results showed that: (1) β 1 , 4 -galactosyltransferase showed similar branch specificity and kinetics for the biantenary substrate during development; (2) GalTase activity in fetal mouse brain was four times higher than that in adult mouse brain and decreased gradually in the course of development; (3) messenger RNA level was highest in fetal mouse and decreased dramatically after birth. However, the contents of mRNA were not parallel to the enzyme activity.