Rats at ages 12–18 days were injected intracranially with 14C-choline and 14C-leucine in the presence and absence of puromycin, then sacrificed at various time intervals, and nerve endings and microsomes were isolated from the cerebra by means of differential and density-gradient centrifugation. The specific radioactivities of lecithin and insoluble proteins increased in nerve endings and decreased in microsomes in the short-term experiments and bcame similar from both particulates after 72 hr from time of administration. Purmoyein exerted an appreciable inhibitory effect on the synthesis of both lipids and proteins from both fractions in vivo, but had a negligible effect on lecithin synthesis in vitro. No differences were found between the properties of Na+-K+-ATPase from the nerve-ending and microsomal fractions. From the relationship between the synthesis of lecithin and proteins in vivo in both particulates and the synthesis of lecithin in vitro, it can be concluded that the synapse can derive its lipids and proteins from the cell body as well as from local synthesis. © 1970, SAGE Publications. All rights reserved.