A high peptidylglycine alpha-amidating mono-oxygenase (PAMase) activity has been measured in the pancreas of neonatal rats. A significant fraction of this activity is contained in the beta cells of the islets of Langerhans and is colocalized with thyrotropin-releasing hormone (TRH) and its precursor in secretory granules. The ontogenetic variation of PAMase activity in the pancrease parallels that of TRH concentrations, suggesting that this enzymatic activity is directly related to TRH biosynthesis. In addition, PAMase activity is able to generate TRH when incubated with less than Glu-His-Pro-Gly, a tetrapeptide present as a repetitive sequence in the TRH precursor. The perinatal evolution of the TRH precursor levels in the pancreas is similar to that of PAMase activity (unpublished results). Thus, the neonatal rat pancreas offers an endocrine model in which the levels of a neuropeptide precursor and an enzyme activity, involved in the posttranslational modification of this precursor, are similarly regulated. Our results suggest also that a fraction of PAMase activity may be produced outside of the beta cells and related to the biosynthesis of COOH-terminally amidated peptide(s) other than TRH. The ontogenetic changes in PAMase activity imply that the synthesis of this peptide(s) is high during the neonatal period, decreasing thereafter.