The in vitro synthesis of DNA as measured by the incorporation of thymidine-2-14C into DNA has been studied for various regions of the infant rat brain. Both intact cerebellum and cell-free extracts of cerebellum from newborn and infant rat brain showed a very rapid rate of DNA synthesis which was highest around 6 days after birth and decreased rapidly thereafter up to 18 days. This DNA synthesis in developing rat brain was strongly inhibited by hydroxyurea but was much less sensitive than was RNA synthesis to inhibition by actinomycin D.