Polypeptides labeled in axons, in the myelin sheath, and in nerve cell bodies with [(35)S]methionine were analyzed separately in cytologically pure microscopic samples after incubation in vitro. Labeled polypeptides were evaluated and compared in (i) the goldfish Mauthner (M) cell axon and its myelin sheath and (ii) dorsal and ventral root axons, corresponding Schwann cell/myelin sheaths, and dorsal root ganglion cells in the rat. In all cases, polypeptides in axonal samples were not labeled in the presence of cycloheximide. Labeling patterns in the M-cell axon and in axons of dorsal and ventral roots were complex, neuronal in character, and distinctive in comparison with the myelin sheath. In all instances, tubulin and actin were labeled. Evidence is adduced that the myelin sheath of the M-cell axon appears also to have a local capacity for endogenous protein synthesis. The results reported here suggest that a number of constituent proteins of slow transport groups in myelinated axons probably turnover during transport and are replaced by local endogenous protein synthesis.