The 5'-untranslated leader of maize (Zea mays) heat-shock protein (hsp) 70 mRNA is required for translational competence during heat shock in protoplasts. When the beta-glucuronidase gene was used as a reporter mRNA, expression at elevated temperatures increased more than 10-fold when the hsp70 leader constituted the 5'-untranslated region. The hsp70 leader did not affect the physical half-life of the mRNA and, therefore, does not function at the level of transcript stability. The maize hsp70 leader was required to escape thermal repression in both maize and tobacco (Nicotiana tabacum) but was less effective in carrot. In addition, mRNAs containing the tobacco mosaic virus untranslated leader (omega) were also efficiently translated during heat shock, data suggesting that the presence of the omega sequence enables the transcript to escape the translational repression that occurs during thermal stress.