In order to investigate an endocrinological effect on metamorposis of the midgut in silkworm, Bombyx mori, 20-hydroxyecdysone and rude extract of brain (PTTH) were injected into pupal isolated abdomens, devoid of prothoracic glands and also in the brainless diapausing pupae with the prothoracic glands (so-called dauer pupae). When ecdysone was assayed abdomens, drastic morphological changes occurred in the midgut. The wet weights of midguts were found decrease as like the normal pupae at metamorphosis. Proteineous changes in epithelial midgut were also observed; pupal specific proteins (PSP) vanished while the adult proteins were newly synthesized. Similar results were also found in the changes of protein in the midgut of dauer pupae by injecting ecdysone and PTTH.