Analysis of synaptosomal plasma membrane from mouse brain by SDS gel electrophoresis reveals a large number of glycoprotein and glycolipid species, as distinguished by periodic acid Schiff or by the distribution of in vivo incorporated 3H-fucose label. The topographical distribution of this carbohydrate in the plasma membrane was examined by labelling intact synaptosomes and isolated membrane with galactose oxidase (E.C. 1.1.2.9.)-3H-NaBH4. In both, gangliosides and a wide spectrum of glycoprotein species were labelled. Extraction of SPM with 0.05 N NaOH at 0 degrees C removed about two-thirds of the membrane's protein. Gel electrophoresis of the solubilized material revealed partial or complete extraction of a large number of polypeptide species; however, all of the glycoproteins, as well as the glycolipid, remained in the insoluble pellet. These results indicate a highly asymmetric arrangement of glycoproteins and glycolipids in SPM; most or all of these species reside on the external surface of the synaptosome, accessible to galactose oxidase. The glycoproteins appear to be more deeply-buried in the lipid bilayer than the other polypeptide species, as has been found in other systems.