The abdominal prolegs of Manduca sexta larvae are eliminated at the onset of metamorphosis. Previous work showed that the prepupal peak of ecdysteroids in the hemolymph causes the dendritic arbors of proleg motoneurons to regress and a stereotyped subset of the motoneurons to die. In the present study we investigated the parameters of ecdysteroid exposure that are important for eliciting these responses by directly infusing 20-hydroxyecdysone (20-HE) into the hemolymph of insects deprived of their own endocrine glands. Doses of 20-HE that were near threshold for evoking regression or death were consistently more effective when infused over a longer duration. Theoretical calculations of hemolymph hormone profiles produced by the infusions support a model of ecdysteroid action in which the hormone concentration must remain above a threshold level for a critical duration of time to be physiologically effective. We further found that segmental location can influence both the metamorphic fate and the hormonal sensitivity of Manduca motoneurons.