We have examined the effect of mild hyperthermia on the pattern of heat shock transcription factor (HSF) binding activity, heat shock protein 70 (hsp70) and hsp30 gene expression and protein denaturation in selected tissues of adult Xenopus namely, heart, hind limb muscle, eye, liver and spleen. In these studies it was found that heart tissue was the most thermally sensitive of all of the tissues examined since maintenance of adult frogs at 26°C resulted in a preferential activation of HSF binding. Thus, heart has a lowered set point temperature for HSF activation compared to the other tissues examined. At 30°C HSF activation was observed in all of the tissues examined. Heart HSF activation at 26°C was correlated with an increase in hsp70 mRNA and Hsp70 protein accumulation. At 28°C the largest amount of hsp70 and hsp30 mRNA accumulation was detected in heart and skeletal muscle compared to other tissues while hsp70 mRNA accumulation was relatively low in spleen and hsp30 mRNA accumulation was not detectable in eyes, liver and spleen. Incubation of adult frogs at 30°C resulted in enhanced hsp70 and hsp30 mRNA accumulation in all of the tissues. Finally, we have used differential scanning calorimetry (DSC) to compare the temperatures at which protein denaturation occurs in heart and liver tissue. The onset of protein denaturation (T0,) occurred approximately 8.5°C lower in heart compared to liver. Also the midpoint of the DSC profile (T½) was approximately 10.4°C lower in heart than in liver. Thus, heart proteins are generally more thermolabile than proteins in liver tissue. Taken together these data suggest that heart is more sensitive than the other tissues examined with respect to moderate increases in environmental temperature.