Abstract Incubation of plasma from the steelhead trout, Oncorhynchus mykiss with porcine pancreatic glandular kallikrein generated bradykinin-like immunoreactivity. The primary structure of the immunoreactive peptide was established as: Lys-Arg-Pro-Pro-Gly-Trp-Ser-Pro-Leu-Arg. This sequence shows two amino acid substitutions (Phe 6→Trp and Phe 9→Leu) compared with mammalian lysyl-bradykinin (kallidin). Bolus intra-arterial injection of the purified peptide produced a strong and sustained vasopressor response in the unanaesthetized trout. The data demonstrate that the kallikrein—kinin system predates the appearance of tetrapods and suggest a role for this system in cardiovascular regulation in fish.