The larval-pupal transformation of Manduca sexta results from an exposure to ecdysteroids in the absence of juvenile hormone (the commitment pulse), followed by a larger exposure to ecdysteroids (the prepupal peak) with a reappearance of juvenile hormone (JH). The prepupal ecdysteroid peak triggers the degeneration of abdominal muscles, and the dendritic regression and death of identified motoneurones. The present experiments examined the role of the commitment pulse in the larval-pupal reprogramming of these cells. The commitment pulse did not overtly affect the muscles and motoneurones, but it switched their hormonal responsiveness; before the commitment pulse, exposure to ecdysteroids in the presence of JH had no effect on the larval cells, whereas after the commitment pulse the same treatment caused regression and death. Thus, JH lost its ability to prevent pupal development. Furthermore, treatment with ecdysteroids in the absence of JH before the commitment pulse promoted pupal development much less effectively than did the same treatment given after the commitment pulse, indicating that the commitment pulse facilitates the subsequent responsiveness to ecdysteroids. Thus, the commitment pulse covertly causes both qualitative and quantitative changes in the hormonal sensitivity of the larval muscles and motoneurones.