Insulin stimulates the translation of ribosomal protein (r-protein) mRNAs and the transcription of rDNA in mouse MM14DZ myoblasts. Analysis of the distribution of S16, L18, and L32 r-protein mRNAs in polysome gradients indicates that the increased translation of these mRNAs in insulin-treated myoblasts is due to the recruitment of mRNAs that were not previously being translated. In contrast, the translational efficiencies of beta-actin, c-myc, and p31 mRNAs are not affected by insulin. Hybridization analysis of RNA transcribed in nuclear run-on reactions indicates that insulin also stimulates the transcription of rDNA. Both the increases in r-protein translation and rDNA transcription occur coordinately and are maximal within 15 min of insulin treatment of myoblasts. However, insulin has no effect on the rate of cell division or the steady state levels of r-protein mRNAs. Surprisingly, after myoblasts differentiate into fibers, insulin does not affect the r-protein mRNA translation or rDNA transcription. These experiments indicate that the synthesis of the macromolecular components of ribosomes is tightly and coordinately controlled in myoblasts.