The action of LiCl on RNA and protein synthesis in Amphibian embryos was studied previously. In this paper the NaSCN effect, which appears in opposition to the LiCl effect, was examined on the same material; this was shown by morphological research as well as by experiments on the protein in solution. Xenopus laevis and Rana esculenta embryos were used and were treated, at late blastula stage, with NaSCN in tap water at increasing concentrations from 1,5 mM to 6.0 mM. After three hours of treatment the rearing mediums were diluted 1 : 100 with tap water and the embryos were maintained overnight in this diluted solution. A first series of experiments of incorporation of $\sideset{^3}{}{\operatorname{H}}$H-uridine in Xenopus treated embryos (embryos treated with NaSCN concentrations higher than 2.5 mM and near to those morphologically active) showed that the quantity of RNA synthetized during the incubation-time was generally superior to the control. On the contrary, at lower concentrations the incorporation was reduced (fig. 1). The sucrose gradients analysis of RNA revealed that the greater quantity of RNA, discovered in extract of 3.0 mM NaSCN treated embryos, was due for the most part to an 18 S fraction and to high molecular weight precursors; the increase of the 28 S fraction was lower (fig. 2). On comparing the results obtained by measuring uridine incorporation in RNA and from the analysis of sucrose gradients, it can be supposed that the larger quantity of RNA present in treated embryos was due to an increase of synthesis. This was also confirmed by the results of experiments of hydrolysis in vitro of NaSCN treated RNA by ribonucléase (fig. 3). A second series of experiments on incorporation of labelled precursors in proteins of Xenopus and Rana embryos treated with NaSCN showed an increased synthesis at low salt concentrations. The highest increase was observed at concentrations of 1.5 mM for Xenopus and 2.5 mM for Rana (fig. 4).