The purpose of this communication is to provide a survey of contempory information concerning the composition, disposition, and functional significance of proteins in or on synaptic membranes derived from rat brain synaptosomes. Special emphasis is placed on their content of glycoproteins, fibrous proteins, proteins susceptible to rapid phosphorylation and dephosphorylation, and receptors for various putative neurotransmitters. Their nature, localization, possible interactions, and potential function in synaptic transmission are discussed.