We have used the P-element-mediated transformation of Drosophila germ line to study the 5' DNA sequences involved in the thermal inducibility of the genes for heat shock proteins hsp23 and 26. The results are strikingly different from those previously obtained in heterologous systems. For hsp23, each successive shortening of the promoter region from 618 to 402, 321 and 263 bp clearly decreased the expression. A construct with only 149 bp was not inducible at all. For hsp26, all the regulatory elements appear to be clustered in the first 350 bp upstream from the cap site. Clones with 171 bp showed a 4- to 10-fold decrease in induction depending on the transformed line, and those with only 52 bp were not expressed. The results suggest that at least three Pelham consensus sequences are required for the full expression of these two genes. The direct involvement of one of these consensus sequences has been assessed: a 6-bp deletion within the proximal element of the hsp26 gene strongly reduced its inducibility. Our results also indicate that X-linked hsp genes exhibit either partial dosage compensation or none at all.