Synaptosomes were prepared from bovine brain by zonal rotor sucrose density centrifugation. While a major fraction of lipid-bound sialic acid is included uniformly within the synaptosomal distribution profile, the sialoglycoproteins and some gangliosides do not follow this pattern. Exposure to extrasynaptosomal calcium results in alterations in the surface labeling properties of some gangliosides and membrane plasmalogens, suggesting that extrasynaptic Ca2+ may influence the conformation of complex lipids in synaptic plasma membranes. The level of intrinsic membrane-associated sialidase activity that liberates sialic acid from these sialoglycoconjugates parallels the synaptosomal buoyant density distribution profile, supporting a view that this enzyme resides in synaptosomal membranes in close association with a sialolipid substrate.