New cell production in the corpus callosum and subependyma of 4 month old mice was analysed by light microscopic radioautography after 3H-thymidine injections. In the subependymal region of the lateral ventricle, about 10% of cells were labeled in mice examined 2 h after 3H-thymidine, and 40 to 50% of cells were labeled after 7 d of isotope infusion. In corpus callosum of mice 2 h after precursor injection, the few cells (0.1 to 0.2%) that were labeled had the appearance of "immature cells", and were presumably incompletely-differentiated neuroglial precursor cells which were preparing to divide. After 7 d of continuous 3H-thymidine administration, more labeled neuroglia (about 2%) were detected in corpus callosum; these newlyproduced cells included several astrocytes and some oligodendrocytes, as well as immature cells. Since the immature cells were the most frequently observed type of dividing cell within the normal adult corpus callosum, it is probable that the new astrocytes and oligodendrocytes were the products of divisions of their respective precursor cells.