The rate of metamorphosis in Manduca appears to be under continuous regulation by the circulating titer of the ecdysteroids. Ecdysteroids promote development during the first third of adult differentiation. We present here several lines of evidence indicating that the role of the ecdysteroids then changes to being inhibitory during the later stages of adult differentiation. Abdomen ligation, which precipitously reduces the levels of ecdysteroids in the abdomen, accelerates the rates of tissue development in this region. This acceleration can be counteracted by ecdysteroid injection or by implantation of prothoracic glands. Infusion of ecdysteroids into insects late in development results in a dose-dependent depression in the rate of subsequent development. The effectiveness of a given dosage of steroid is dependent on the developmental stage, with older animals being more affected. Last, the normal ecdysteroid titer declines in a stepwise fashion over the last 3 days of development and these steps are paralleled by a drop-off in the effectiveness of abdomen ligation over this same period. It is concluded that this effect of the ecdysteroids late in development provides a mechanism to ensure that the various tissues of the insect complete metamorphosis in a coordinated fashion.