Chromosomal RNA synthesis of the three heat shock puffs 87A, 87C and 93D examined in trisomic 3L (T3L) and trisomic 2L (T2L) strains of Drosophila melanogaster and compared with that of Oregon R + and compound stocks of the 3rd [C(3L) VGI ru st; F(3R) VDI es/F(3R) VDI es] and 2nd [C(2L) SHI+; F(2R) bw] chromosomes. The activity at two different temperatures (22° and 37° C) was monitored by [3H]uridine autoradiography. The results that while in the control T3L and T2L nuclei (22° C), the 93D puff shows a higher level of transcriptional activity than in the Oregon R+ or compound stocks used as controls, in T3L and T2L nuclei from heat-shocked sets (37° C), 93D does not show further induction compared with heatshocked controls, and the 87C puff is 2.8 times more active than the 87A puff. These results suggest that there may be a direct functional and/or regulatory relationship between the activity of 93D and certain sequence specific activity of 87C, and that an optimum level of activity delimits the activity of the heat shock puff 93D in D. melanogaster. © 1990 Springer-Verlag.