AbstractCarbohydrate‐containing macromolecules of brain were substantially labeled after intracerebral injection of C14‐glucosamine. The “half‐life” of the labeled macromolecules, as measured in these experiments, was about one week. Acetoxycycloheximide inhibited incorporation of C14‐glucosamine into these substances. This appears to be secondary to inhibition of synthesis of polypeptide chains which act as “acceptors” for carbohydrate residues. Acetoxycycloheximide inhibited incorporation of C14‐glucosamine (and its derivatives) into the soluble carbohydrate‐containing macromolecules of nerve endings far less than that of the other subcellular fractions of brain. This suggests that the polypeptide “acceptors” of carbohydrate residues at nerve endings are not synthesized locally but are transported to the nerve endings. Final incorporation of carbohydrates into carbohydrate‐containing macromolecules occurs at nerve endings. The “turnover” of these substances in nerve endings was found to be relatively rapid. Evidence suggesting reversible incorporation and removal of some carbohydrate residues of carbohydrate‐containing macromolecules of brain was found and the possible regulatory rol