Effects of ecdysteroids on two components of the juvenile hormone (JH) biosynthetic pathway were determined during the early stage of fifth instar larvae of tobacco hornworm, Manduca sexta. When ecdysed fifth instars were fed a diet containing nonsteroidal ecdysteroid agonist, RH-5992, the normal decrease of the JH biosynthetic activity in corpora allata (CA) was prevented until day 2 larvae. In addition,20-hydroxyecdysone (20E) directly stimulated CA activity in a long-term culture for 24 hours, and its effect was not influenced by the simultaneous treatment with M. sexta allatotropin (Manse-AT). The Manse-AT mRNA-3, which is one alternatively spliced mRNA among 3 isoforms of the Manse-AT gene, was exclusively present in the terminal abdominal ganglion (TAG) of the fifth instar larvae(Lee and Horodyski, 2002). The level of Manse-AT RNA-3 was increased in the TAG of the larvae which ingested RH-5992. These results suggest that the levels of hemolymph ecdysteroids exert a tropic activity in CA and the TAG and could be associated with the control mechanism of internal physiological changes, such as the nutrientrestricted condition.