The skin of Xenopus laevis, because of its high concentrations of amidated peptides, is an excellent model for the study of neuropeptide biosynthesis. In this paper we determined levels of the mRNAs which encode cerulin and characterized the enzymatic activity which likely produces the carboxyl terminal α-amide moiety of cerulein. This enzyme is stimulated by the addition of CuSO4 and ascorbate and requires the presence of molecular oxygen and is thus similar to the peptidyl-glycine α-amidating monooxygenase (PAM) described in mammals. Hybridization analysis showed that three cerulein mRNAs of different molecular sizes are present in varying proportions in the skin of individual frogs. Injection of norepinephrine into the dorsal lymph sac of Xenopus caused immediate secretion and depletion from skin of PAM activity but did not cause a comparable immediate decrease in skin levels of cerulein mRNA or total RNA. By 2 weeks after norepinephrine injection, cerulein mRNA levels increased to fourfold over control levels. By 4 weeks, skin PAM activity was almost restored to control levels.