AbstractEcdysone is the molting hormone of insects. It is a steroid; its chemistry and physiology are briefly reviewed. One of its most interesting physiological actions is the production of “puffs” in the salivary gland chromosomes of the midgeChironomus. Since puffs are generally believed to represent activity structures of genes, the implications of gene activation have been investigated. In the blowflyCalliphora, the synthesis of messenger RNA has been demonstrated; this messenger carries the information for the enzyme dopa decarboxylase. This enzyme is inducedin vivoby ecdysone; this induction can be inhibited by actinomycin, puromycin, and other inhibitors of RNA and protein synthesis. Dopa decarboxylase is one of the key enzymes in the process of sclerotization, in which tyrosine metabolites are incorporated into the cuticle, resulting in tanning. Thus, all steps from gene activation through RNA and protein synthesis to the final physiological response have been demonstrated experimenta