Cuticular sclerotization in insects requires dopamine derivatives and thus the presence of dopa decarboxylase (DDC), the enzyme which converts dopa to dopamine. During the last half of the larval molt of the tobacco hornworm, Manduca sexta, beginning at 16 hr after head capsule slippage, the epidermal DDC activity increased fourfold. By contrast, allatectomized larvae which were destined to produce a melanized cuticle showed a sevenfold increase. This increase in DDC activity was prevented by infusion of 20-hydroxyecdysone (20HE) into the larva, indicating that the fall of the ecdysteroid titer is necessary for the increase. In vitro 20HE also prevented the increase in a dose-dependent manner when the epidermis was explanted at 16 hr after head capsule slippage but had less effect on epidermis explanted 3 hr later. Both 5 micrograms/ml alpha-amanitin and 100 micrograms/ml cycloheximide also prevented the increase. Application of juvenile hormone I showed that the critical period for determination of the level of the later increase in DDC activity was about 4 hr after head capsule slippage at the peak of the ecdysteroid titer. Apparently then the rise and fall of ecdysteroid regulate different aspects of DDC synthesis, the rise determining its later appearance and the fall timing this appearance.