The skin of the European common frog, Rana temporaria, contains a very high concentration of bradykinin (BK) but the mode of biosynthesis of the peptide is unknown. In addition to BK, we have isolated from an extract of R. temporaria skin the metabolites [des-Arg9]BK and [hydroxyprolyl3]BK. Peptides were also isolated that represent partially processed intermediates in the biosynthetic pathway to BK and comprise BK extended from its N-terminus by -Gly-Val-Ile-Pro-Leu-Leu and three peptides comprising BK extended from its C-terminus by -Ile-Ala, by -Ile-Ala-Pro-Ala-Ser-Thr-Leu, and by -Ile-Ala-Pro-Ala-Ser-Ile-Leu. The isolation of two C-terminally extended BK-containing peptides differing by a single amino substitution indicates that the biosynthetic precursor of frog skin BK may contain more than one copy of the BK sequence and/ or more than one gene encoding BK is expressed. The structures of the biosynthetic intermediates suggest that BK in frog skin is generated by the action of cellular endoproteinase(s) cleaving at the site of monobasic residues rather than by the action of the kallikrein-kinin system.