Incubation of heat-denatured plasma from six species occupying different evolutionary positions within the Sarcopterygian lineage [the dipnoan,Protopterus annectens(African lungfish); the urodele,Amphiuma tridactylum(three-toed amphiuma); the colubrid snakes,Pituophis melanoleucus sayi(bullsnake) andMasticophis flagellum(coachwhip); and the lizardsHeloderma suspectum(Gila monster) andVaranus Grayi(Gray's monitor)] with trypsin generated bradykinin-related peptides that were detected by radioimmunoassay using an antiserum raised against mammalian bradykinin (BK). The peptides were purified by HPLC and their primary structures were established as lungfish [Tyr1,Gly2,Ala7,Pro8]BK, amphiuma [Phe1,Ile2,Leu5]BK, bullsnake and coachwhip [Val1,Thr6]BK, Gila monster [Leu2,Thr6]BK, and Gray's monitor [Thr6]BK. Monitor BK is identical to the peptide generated in turtle and alligator plasma and coachwhip/bullsnake BK shows one amino acid substitution (Ala1→Val) compared with the peptide generated in the plasma of the python. The data provide further evidence for the widespread occurrence of a kallikrein–kininogen system in nonmammalian vertebrates but indicate that the primary structure of BK has been poorly conserved during evolution.