The subcellular RNA and protein metabolism have been studied in emetine-treated rats. Emetine treatment for a period of 10 days reduced the incorporation in vivo of [14C]-leucine into proteins of nuclear fraction of liver. This was accompanied by a reduction in the nuclear protein content. The mitochondrial fraction of heart exhibited an increase in the protein content after emetine treatment. This was, however, not accompanied by an increased rate of incorporation into its protein. On the other hand, the nuclear fraction of heart of emetine-treated rats showed a diminution in the rate of incorporation into its protein without having been reduced in protein content. The mitochondrial and microsomal RNA of liver and the microsomal RNA of heart responded to emetine treatment by showing increased levels. The liver RNase activity was reduced after emetine treatment, while the heart RNase remained independent of the treatment. It has been suggested that both synthesis and breakdown of liver proteins are reduced by emetine treatment. The synthesis of liver nuclear proteins was supposedly affected more than was its breakdown by emetine. The synthesis of rapidly turningover proteins of heart nuclear fraction and the catabolism of slowly turningover proteins of its mitochondrial fraction are thought to be reduced by the emetine treatment.