Abstract We have demonstrated that a wheat germ extract, primed with yeast RNA, synthesizes at least thirty five yeast ribosomal proteins. These proteins are all coded by mRNA molecules which contain poly(A). By quantitative analysis of the products formed under limiting mRNA concentrations, we have measured the level of mRNA for the individual ribosomal proteins under various conditions. In confirmation of studies in vivo (Gorenstein and Warner, 1976), we find that the synthesis of mRNA for the different ribosomal proteins is coordinately regulated. Within 5 min after wild-type cells are shifted from 23°C to 36°C, the synthesis of mRNA for most of the ribosomal proteins ceases. The mRNA for ribosomal proteins that is present decays with a half-life of 8–15 min. The synthesis of mRNA for nonribosomal proteins is relatively unaffected. After 15–20 min, the synthesis of mRNA for ribosomal proteins resumes, and by 60 min, the production of ribosomal components has equilibrated at the increased temperature. In a set of mutants temperature-sensitive ( ts ) for ribosome synthesis, the resumption of synthesis of mRNA for ribosomal proteins never occurs, and the cells are thus unable to form new ribosomes at the restrictive temperature, although transcription of ribosomal precursor RNA continues. We conclude that the synthesis of ribosomal proteins in yeast is coordinately regulated at the level of transcription. Furthermore, transcription of ribosomal precursor RNA can be uncoupled from the transcription of mRNA for ribosomal proteins.