The turnover of nucleic acids and proteins in the central nervous system has been explored by autoradiography following the subarachnoid injection of tagged precursors. Nuclear PNA of neurons and oligodendrocytes becomes radioactive earlier than cytoplasmic PNA after injection of adenine-C(14) and orotic-C(14) acid. By 24 hours following injection, cytoplasmic PNA is radioactive. Radioactivity persists with little decrease for as long as 51 days after an injection of adenine-C(14). The cells of the ependymal lining, choroidal plexus, leptomeninges, blood vessel walls, and Schwann cells also exhibit radioactivity in PNA as judged by the loss of radioactivity following ribonuclease digestion. From the 3rd day on, increasing numbers of the aforementioned cells, with the exception of nerve cells, exhibit ribonuclease-resistant nuclear radioactivity which is abolished by deoxyribonuclease. This radioactivity indicates labelling of nuclear DNA. Following the intrathecal injection of methionine-S(35) and glycine-2-H(3), nerve cells, oligodendrocytes, cells of ependymal lining, choroidal plexus, leptomeninges, blood vessels, and Schwann cells become radioactive. Nerve cells lose most of their radioactivity within a few hours, first from the cytoplasm and later from the nucleus. Other cell types retain their radioactivity for considerable periods of time. Although astrocytes, microglia, and satellite cells of sensory ganglia do not appear to incorporate labelled precursors into nucleic acids or proteins, reacting phagocytic microglia actively take up labelled amino acids. These results are discussed with particular reference to PNA and protein turnover in nerve cells, oligodendrocytes, and Schwann cells. It is believed that these metabolic activities in neurons are concerned in part with the elaboration of axoplasmic proteins. The nucleoprotein metabolism of oligodendrocytes and Schwann cells may be related to myelin biosynthesis both in the immature and the mature nervous system.