Nucleic acid synthesis in cleaving embryos of Xenopus laevis has been investigated using autoradiographic and biochemical methods on eggs and embryos labelled by micro-injection of $^{3}$H-nucleosides. During the first few hours after fertilization extremely small amounts of RNA are synthesized, probably in the cytoplasm. Towards the end of cleavage a marked increase in RNA synthesis coincides with a reduction in the frequency of DNA synthesis. In agreement with the results of others, nuclear RNA synthesis can first be detected at this stage, and consists primarily of heterogeneous RNA. Transfer RNA synthesis commences soon after the onset of nuclear heterogeneous RNA synthesis, and ribosomal RNA is not synthesized until gastrulation. Nuclei from neurula embryos, which synthesize each of the above classes of RNA, have been transplanted to enucleated unfertilized eggs. Immediately after transplantation all nuclear RNA synthesis ceases, but as the nuclear transplant embryos pass through cleavage and gastrulation each main class of RNA starts to be synthesized in turn at the same time and in the same sequence as happens in embryos derived from fertilized eggs. These experiments show that egg cytoplasm contains components independently responsible for the repression and subsequent initiation of synthesis of each main class of RNA, and contribute to an analysis of the mechanism by which gene activity is controlled during early animal development.