During heat shock of chicken reticulocytes the synthesis of a single heat shock protein, HSP70, increases greater than 10-fold, while the level of HSP70 mRNA increases less than 2-fold during the same period. Comparison of the in vivo levels of HSP70 and beta-globin synthesis with their mRNA abundance reveals that the translation of HSP70 mRNA is repressed in normal reticulocytes and is activated upon heat shock. In its translationally repressed state HSP70 mRNA is functionally associated with polysomes based on sedimentation analysis of polysomes from untreated or puromycin-treated cells and by analysis of in vitro "run-off" translation products using isolated polysomes. Treatment of control and heat shocked cells with the initiation inhibitor pactamycin reveals that elongation of the HSP70 nascent peptide is not completely arrested, but is slower in control cells. Furthermore, the inefficient translation of HSP70 mRNA in vivo is not due to the lack of an essential translation factor; HSP70 mRNA is efficiently translated in chicken reticulocyte translation extracts as well as in heterologous rabbit reticulocyte extracts. Our results reveal that a major control point for HSP70 synthesis in reticulocytes is the elongation rate of the HSP70 nascent peptide.