Puromycin inhibited protein synthesis in control cultured rat bone marrow cells by 98 %, and had only a slightly inhibitory action on RNA synthesis. Erythropoietin-stimulated RNA synthesis was inhibited completely. Cycloheximide inhibited control protein synthesis to the same extent as puromycin and caused significantly more inhibition of control RNA synthesis, but had no effect on erythropoietin-stimulated RNA formation. There was no difference in the effects of the two inhibitors on the synthesis of cytoplasmic or nuclear proteins, and both inhibited erythropoietin-stimulated protein formation. Prior action of cycloheximide on marrow cells did not prevent the inhibition of induced RNA synthesis caused by puromycin. These data indicate that the early hormone action on RNA synthesis does not require protein synthesis.