Insect molting is elicited by a class of polyhydroxylated steroids, ecdysteroids, that originate in the prothoracic glands. Ecdysteroid synthesis in the prothoracic glands is controlled in large measure by a peptide hormone from the brain, prothoracicotropic hormone (PTTH), which exists in two forms and is released into the general circulation as a result of environmental and developmental cues. The means by which PTTH activates the prothoracic glands has been examined at the cellular level and the data reveal the involvement of (...) cAMP, calcium, calmodulin, cAMP‐dependent protein kinase and the ultimate phosphorylation of a 34 kDa protein tentatively identified as ribosomal protein S6. (shrink)