Regulation of the haemolymph titres of ecdysteroids and the juvenile hormones (JH) during larval-pupal development of the tobacco hornworm, Manduca sexta, involves the interendocrine control of the synthesis of each hormone by the other. Temporal relationships between the ecdysteroid titre peaks in the fourth and early fifth larval instar and the increases in corpora allata (CA) activity at these times suggests that ecdysteroids are evoking the increases. Incubation of brain-corpora cardiaca-corpora allata (Br-CC-CA) complexes and isolated CA from these stages with 20-hydroxyecdysone (20-HE) revealed that 20-HE stimulates CA activity and that it does this indirectly via the Br-CC. The resulting increase in the JH titre after the commitment (first) peak in the fifth instar stimulates the fat body to secrete a factor which appears to be the same as a haemolymph stimulatory factor for the prothoracic glands. This moiety acts as a secondary effector that modulates the activity of the prothoracic glands and thus the ecdysteroid titre. These findings together have begun to elucidate the mechanisms by which the principal developmental hormones in the insect interact to regulate postembryonic development. © 1986.