Bradykinin (BK) isolated from plasma of the African lungfish, Protopterus annectens, contains four amino acid substitutions compared with BK from mammals (Arg 1 ->Tyr, Pro 2 ->Gly, Pro 7 ->Ala, Phe 8 ->Pro). Bolus intra-arterial injections of synthetic lungfish BK (1-1000 pmol/kg body wt.) into unanaesthetised, juvenile lungfish (n=5) produced a dose-dependent increase in arterial blood pressure and pulse pressure. The maximum pressor response occurred 2-3 min after injection and persisted for up to 15 min. The threshold dose producing a significant (P<0.01) rise in pressure was 50 pmol/kg and the maximum increase, following injection of 300 pmol/kg, was 9.3+/-2.3 mmHg. Injection of the higher doses of lungfish BK produced a significant (P<0.05) increase in heart rate (2.8+/-0.8 beats/min at 100 pmol/kg). In contrast, bolus intra-arterial injections of mammalian BK, in doses up to 1000 pmol/kg, produced no significant cardiovascular effects in the lungfish. The data support the existence of a functioning kallikrein-kinin system in the lungfish and demonstrate that the ligand-binding properties of the receptor(s) mediating the cardiovascular actions of lungfish BK are appreciably different from mammalian B1 and B2 receptors.