A peptide with bradykinin (BK)-like immunoreactivity was isolated from an incubate of heat-denatured eel plasma with porcine pancreatic kallikrein. The purified peptide had the following amino acid sequence: Arg-Arg-Pro-Pro-Gly-Ser-Trp-Pro-Leu-Arg. This decapeptide, named eel [Arg⁰]BK, was identical to two previously identified BK homologs from cod and trout. High conservation of the BK sequence among distant teleost species suggests an important function in this vertebrate group. Bolus intra-arterial injections of eel [Arg⁰]BK, BK, and [Arg⁰]-des-Arg⁹-BK (1-10 nmol/kg) caused significant (P < 0.05) inhibition of drinking in seawater-adapted eels. The potency of the inhibition was ranked in the following order: [Arg⁰]BK > [Arg⁰]-des-Arg⁹-BK = BK. The BK peptides also produced an immediate, transient increase followed by a sustained increase in arterial blood pressure and an initial decrease followed by an increase in heart rate. Strong tachyphylaxis occurred for the cardiovascular effect but not for the antidipsogenic effect. The order of the potency of the cardiovascular actions, [Arg⁰]BK > BK > [Arg⁰]-des-Arg⁹-BK, was different from that of the antidipsogenic action. Slow infusions of eel [Arg⁰]BK in the dose range 1-1,000 pmol · kg¹ · min¹ produced concentration-dependent inhibition of drinking without changes in arterial pressure, plasma osmolality, and hematocrit. At the infusion rate of >100 pmol · kg¹ · min¹, plasma concentrations of angiotensin II, a potent dipsogenic hormone in eels, increased, suggesting an interaction of the kallikrein-kinin and renin-angiotensin systems. In mammals, BK is dipsogenic and vasodepressor, so that our data demonstrate opposite effects on fluid and cardiovascular regulation of BK in the eel and suggest a new physiological role for the kallikrein-kinin system in teleost fish.