Changes in populations of translatable messenger ribonucleic acids (mRNA's) after heat shock of Saccharomyces cerevisiae were examined and found to correlate very closely with transient alterations in patterns of in vivo protein synthesis. Initial changes included an increase in translatable species coding for polypeptides synthesized during heat shock; this increase was found to be dependent on transcription but did not require ongoing protein synthesis. A decrease was observed in the level of translatable mRNA's coding for polypeptides whose synthesis was repressed after heat shock. This decrease was much more rapid than can be explained solely by termination of transcription. Requirements for this rapid loss of RNA from the translatable pool included both transcription and an active rna1 gene product but not protein synthesis. After the initial changes in translatable RNA induced by heat shock, the patterns of both in vivo and in vitro translation products began to revert to the preshock levels. This recovery period, unlike the earlier changes, was dependent upon a requisite period of protein synthesis.