The naturally occurring amphibian skin peptides dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) are highly potent and selective agonists at the mu- and the delta-opioid receptors, respectively. For peptides synthesized by animal cells, they have a rather peculiar structural feature of containing a D-amino acid residue in their sequence which imparts biological activity on them. The cloned cDNA encoding the prodermorphin precursor contains the usual alanine and methionine codons at positions where D-alanine and D-methionine are present in the mature products. In this study, dermorphin precursor was characterized in extracts from amphibian skin by antisera recognizing distinct epitopes within the predicted structure of pro-dermorphin. Proteolytic digestion of purified endogenous pro-dermorphin generated a peptide containing a D-alanine in position 2, identified as prepro-dermorphin-(80-89), i.e. Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-Gly-Glu-Ala. In addition, analysis of skin extracts by enzyme immunoassays coupled to high performance liquid chromatography separations revealed the presence of, besides dermenkephalin, novel dermenkephalin-related peptides, i.e. [L-Met2]dermenkephalin, dermenkephalin-OH, and [Met(O)6]dermenkephalin. [L-Met2]dermenkephalin was present in frog skin in a concentration of about 100 times that of dermenkephalin. These observations confirm that, despite the presence of D-amino acid residues, dermorphin and dermenkephalin are genuine products of post-translational processing of a ribosomally made precursor. They suggest that D-Ala and D-Met develop from a dehydrogenation/hydrogenation stereoinversion of their corresponding L isomers incorporated into pro-dermorphin, a process that occurs with low efficiency at an early stage of biosynthesis.