We studied RNA synthesis in Xenopus laevis embryos at the gastrula stage, using both 001 M LiCl concentration, which causes malformations, and lower concentrations to 0.0025 M that allow normal development. A preliminary series of observations on $\sideset{^{14}}{_2}{\operatorname{CO}}$ incorporation in treated Xenopus embryos revealed that LiCl concentrations below 0.01 M increase the labelled RNA to 38%. On the contrary, in embryos treated with LiCl concentrations above 0.01 M there is a lower incorporation than in the controls. Therefore the sucrose gradient technique was used to determine what RNA fractions cause this increase. Sedimentation patterns show the three RNA fractions 4, 18 and 28 S. There are no appreciable differences between optical density in treated and in control embryos. However, in embryos treated with 0.0025 M. LiCl there is considerable incorporation in each RNA species. As the concentration is increased (up to LiCl 0.01 M), the incorporation decreases, although it is still greater than in the controls. Moreover, embryos treated with 0.0025-0.01 M LiCl present a marked peak of labelled RNA between the two fractions identifiable as light ribosomal RNA (18 S) and heavy ribosomal RNA (28 S). Experiments were carried out to discover whether the labelled RNA accumulation was due to reduced breakdown or to increased synthesis. For this purpose the effects of LiCl on RNA breakdown in presence of ribonucléase were studied. In all concentrations tested in these experiments, lithium inhibits RNA enzymatic hydrolysis.