Ecdysteroid titer and dopa decarboxylase (aromatic-L-amino-acid carboxy-lyase, EC 4.1.1.28) activity were determined throughout the life cycle of Drosophila melanogaster. Five peaks in the amount of hormone were observed, which preceeded five dopa decarboxylase peaks by times ranging from 5 to 58 hr. Late in the third instar the hormone and enzyme maxima are nearly coincident. The increase in enzyme activity observed at this time is paralleled by an increase in translatable dopa decarboxylase mRNA. To obtain evidence that ecdysterone induces the appearance of this mRNA we made use of the temperature-sensitive ecd1 mutant. Garen et al. [Garen, A., Kauvar, L. & Lepesant, J.-A. (1977) Proc. Natl, Acad. Sci. USA 74, 5099-5103] have shown that when third instar mutant larvae are kept at 29 degrees C, the ecdysteroid titer remains low. In such larvae we show that the normal increase in dopa decarboxylase activity fails to appear, and no translatable dopa decarboxylase mRNA can be detected. Exogenous feeding of ecdysterone to these larvae results in a rapid synthesis of dopa decarboxylase in the epidermal cells. In addition, a parallel increase in translatable dopa decarboxylase mRNA occurs, which may be a primary response of these target cells to ecdysterone.