1. Kinetic studies of the incorporation of D-glucosamine-1-14C into microsomal sub-fractions of rat liver revealed that C-fraction (0.26% DOC extract) contained the precursors of plasma glucoproteins in addition to membrane glycoproteins, whereas M-fraction (0.5% DOC extract) was mainly composed of membrane glycoproteins. 2. Two decay slopes were obtained for the membrane glycoproteins during a period from 24 hr to 183 hr after the isotope injection. Half-lives of the slower phases estimated from the slopes after 75 hr were 58–94 hr, the values being about the same as those obtained by labelling proteins with amino acid. The existence of two decay slopes which was not observed using amino acid to label the proteins suggested that the carbohydrate and polypeptide moieties of glycoproteins turned over independently, in an earlier phase of metabolism. 3. D-Glucosamine-l-14C was incorporated into glycoproteins exclusively as glucosamine and sialic acid. The specific activity of the protein bound glucosamine was higher than that of sialic acid during the first 1.5 hr, but thereafter the ratio of specific activity of sialic acid to hexosamine increased gradually, reaching values of 2.6–2.9 for C-fraction and 1.7–1.8 for M-fraction after 24 hr. These facts indicated the metabolic heterogeneity of the carbohydrate chains of microsomal glycoproteins.