Recent studies elucidating the composition and structure of synaptosomal plasma membrane are reviewed and discussed. The chemical composition of this membrane has been very well characterized; it is similar in a general manner to that of other membrane, but shows several distinctive features. The structure of the membrane, including the nature of protein-lipid interactions and the lateral and topographical arrangements of these components, has been less intensively studied, but the available data indicate many features of organization in common with other systems. A close correspondence of synaptosomal plasma membrane composition and structure with that of internal neuronal components is pointed out, and on the basis of it and other observations a model of membrane synthesis is proposed. It is concluded that further studies of this membrane should focus on identifying local differences in membrane organization, the mechanisms by which they are brought about, and their role in synaptic transmission.