The heat shock (hs) response during plant growth and development was analyzed in tobacco and Arabidopsis using chimaeric beta-glucuronidase reporter genes (hs-Gus) driven by a soybean hs promoter. Fluorimetric measurements and histochemical staining revealed high Gus activities in leaves, roots, and flowers exclusively after heat stress. The highest levels of heat-inducible expression were found in the vascular tissues. Without heat stress, a developmental induction of hs-Gus was indicated by the accumulation of high levels of Gus in transgenic tobacco seeds. There was no developmental induction of hs-Gus in Arabidopsis seeds. In situ hybridization to the RNA of the small heat shock protein gene Athsp17.6 in tissue sections revealed an expression in heat-shocked leaves but no expression in control leaves of Arabidopsis. However, a high level of constitutive expression of hs genes was detected in meristematic and provascular tissues of the Arabidopsis embryo. The developmental and tissue-specific regulation of the hs response is discussed.