Abstract—Synaptosomes were isolated from cerebral cortex of young rats and incubated with14C‐labelledl‐leucinein vitro. Amino acid incorporation into proteins of the synaptosomal cytoplasm, mitochondria and membrane components was observed. There was no incorporation into proteins of the vesicles. The protein‐synthesizing system was not stimulated by the addition of either ATP or an ATP‐generating system. ATP at all concentrations was inhibitory.Two different protein‐synthesizing systems operate in the synaptosome. One, sensitive to inhibition by chloramphenicol and related antibiotics, is found in the mitochondrial subfraction and the other, inhibited by cycloheximide, is located either in the membrane components or the synaptosomal cytoplasm. This second system resembles the eukaryotic ribosomal system in its sensitivity to cycloheximide. Both the synaptosomal soluble fraction and the synaptosomal membrane fractions were shown previously to contain RNA. This RNA could function in protein‐synthesizing mechanisms in the synaptosome. These results deomonstrate that protein is synthesized in axonal components and show that it is unnecessary to postulate that all axonal protein is supplied by somat