The postnatal developmental profiles of the protein-bound sialic acid content and the activities of CMP-N-acetylneuraminic acid (CMP-AcNeu) synthetase (EC 2.7.7.43) and glycoprotein sialyltransferase, enzymes involved in sialoglycoprotein biosynthesis, were estimated in the rat brain. The sialoglycoprotein level appeared to increase 2-fold from birth to about 20 days of age, which correlates with the outgrowth of the cell processes in this period. In contrast, the activities per g wet tissue of the enzymes were highest at birth and showed decreasing tendencies during maturation. This revealed that, at least at a certain stage during development, the sialyltransferase is present in structures other than synaptic membranes, since the new-born rat brain is devoid of these membranes. The developmental profile of the endogenous, sialic acid accepting molecules was very similar to that of the sialoglycoproteins. It was concluded that cerebral sialoglycoprotein biosynthesis during postnatal development is not limited by the activities of the synthetase and the transferase, but may largely depend on the production of the endogenous acceptors, which are presumed to be the natural precursors for the sialoglycoproteins.