The localization of the α subunit of the S-100 protein (S-100α) and β subunit (S-100β) was studied in the peripheral nervous system of the rat. In peripheral nerves, S-100α and S-100β were found in the cytoplasm of Schwann cells. Axons were positively stained in part by S-100α and almost totally by S-100β. In the dorsal root ganglia, S-100β was found in satellite cells and their processes and in some neurons. S-100β was found in more of the large neurons, but almost all of the small neurons were negative for S-100β. In the anterior horn cells, S-100β staining was stronger than that of S-100β. In Schwann cells, both S-100β and S-100β were present on the rough endoplasmic reticulum, free ribosomes, and nucleus, as seen by electron microscopy. The S-100α and S-100β in axons were associated with microtubules and neurofilaments.