AbstractThe up‐ and down‐regulation of thesalivary gland secretion protein(Sgs) genes during the third larval instar ofDrosophila melanogasterare controlled by fluctuations of the titre of the steroid hormone 20‐hydroxyecdysone (20E). Induction of these genes by a low hormone titre is a secondary response to 20E mediated by products of 20E‐induced ‘early’ genes. Surprisingly, in the case of theSgs‐4 gene this response also requires a direct contribution of the 20E‐receptor complex. A model is presented which proposes that theSgsgenes, and other 20E‐regulated genes with similar temporal expression profiles, are regulated by complex hormone response units. The hormonal signal is effectively transmitted by these response units only after binding of additional factors, e.g. secretion enhancer binding proteins, which act together in a synergistic manner with the 20E receptor and early gene products to establish a stage‐ and tissuespecific