When Drosophila cells are shifted from 25 degrees C to 37 degrees C, the synthesis of a small group of proteins (the heat shock proteins or HSPs) is rapidly induced, while most preexisting synthesis is repressed. On return to normal growing temperatures, synthesis of HSPs is gradually repressed and normal synthesis is restored. We show that production of HSP 70 (the major heat-induced protein in these cells) is quantitatively correlated with the degree of stress. The level of synthesis is controlled both transcriptionally and posttranscriptionally through repression of HSP 70 mRNA synthesis and destabilization of HSP 70 transcripts. These regulatory mechanisms depend upon the accumulation of the HSPs themselves; when the production of functional HSPs is blocked, HS transcription continues and HS mRNAs are stable, accumulating in vast quantities; if the block is released, a specific quantity of functional HSP must accumulate before HS transcription is repressed and preexisting HS mRNAs are destabilized. Evidence is also presented that indicates that the same quantity of HSP 70 is required to release the block in normal protein synthesis.