1. The developmental program for metamorphosis is determined in third instar larvae of Calliphora and Sarcophaga very early after ecdysis. The process is activated by feeding and can be delayed for many days by starvation.2. High doses of ecdysterone (20 µg per larva) injected just after ecdysis or in starved larvae cause a precocious molt and the formation of supernumerary larvae. The same treatment of larvae a few hours after ecdysis leads to larva-puparium intermediates displaying the mosaic of tanned areas in the old cuticle. Larvae older than 24 hours after ecdysis responded only by forming small puparia.3. Small doses of ecdysterone (less than 5 µg per larva) do not cause a complete molt but activate the "gene-set switching mechanism" for programming of the metamorphosis development. The degree of such a covert effect can be revealed by repeated injections of ecdysterone which lead to formation of more or less advanced larva-puparium intermediates.